lib/sha256: Remove it (#9643)

### Purpose

Remove the `lib/sha256` package, because it's no longer necessary. Go's
standard library now has the same performance and is on par with
`sha256-simd` since [Since Go
1.21](https://github.com/golang/go/commit/1a64574f42b95594cf9c8a12e9ca13d75585429c).
Therefore using `sha256-simd` has no benefits anymore.

ARM already has optimized sha256 assembly code since
https://github.com/golang/go/commit/7b8a7f8272fd1941a199af1adb334bd9996e8909,
`sha256-simd` published their results before that optimized assembly was
implemented,
https://github.com/minio/sha256-simd/commit/f941fedda826b68a196de2e0a9183e273ec0cb91.
The assembly looks very similar and the benchmarks in the Go commit
match that of `sha256-simd`.

This patch removes all of the related code of `lib/sha256` and makes
`crypto/sha256` the 'default'.

Benchmark of `sha256-simd` and `crypto/sha256`:
<details>

```
cpu: AMD Ryzen 5 3600X 6-Core Processor
                │  simd.txt   │               go.txt                │
                │   sec/op    │    sec/op     vs base               │
Hash/8Bytes-12    63.25n ± 1%    73.38n ± 1%  +16.02% (p=0.002 n=6)
Hash/64Bytes-12   98.73n ± 1%   105.30n ± 1%   +6.65% (p=0.002 n=6)
Hash/1K-12        567.2n ± 1%    572.8n ± 1%   +0.99% (p=0.002 n=6)
Hash/8K-12        4.062µ ± 1%    4.062µ ± 1%        ~ (p=0.396 n=6)
Hash/1M-12        512.1µ ± 0%    510.6µ ± 1%        ~ (p=0.485 n=6)
Hash/5M-12        2.556m ± 1%    2.564m ± 0%        ~ (p=0.093 n=6)
Hash/10M-12       5.112m ± 0%    5.127m ± 0%        ~ (p=0.093 n=6)
geomean           13.82µ         14.27µ        +3.28%

                │   simd.txt   │               go.txt                │
                │     B/s      │     B/s       vs base               │
Hash/8Bytes-12    120.6Mi ± 1%   104.0Mi ± 1%  -13.81% (p=0.002 n=6)
Hash/64Bytes-12   618.2Mi ± 1%   579.8Mi ± 1%   -6.22% (p=0.002 n=6)
Hash/1K-12        1.682Gi ± 1%   1.665Gi ± 1%   -0.98% (p=0.002 n=6)
Hash/8K-12        1.878Gi ± 1%   1.878Gi ± 1%        ~ (p=0.310 n=6)
Hash/1M-12        1.907Gi ± 0%   1.913Gi ± 1%        ~ (p=0.485 n=6)
Hash/5M-12        1.911Gi ± 1%   1.904Gi ± 0%        ~ (p=0.093 n=6)
Hash/10M-12       1.910Gi ± 0%   1.905Gi ± 0%        ~ (p=0.093 n=6)
geomean           1.066Gi        1.032Gi        -3.18%
```

</details>


### Testing

Compiled and tested on Linux.

### Documentation

https://github.com/syncthing/docs/pull/874

cmd/syncthing/crash_reporting.go

@@ -9,6 +9,7 @@ package main
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
 	"fmt"
 	"net/http"
 	"os"
@@ -16,8 +17,6 @@ import (
 	"sort"
 	"strings"
 	"time"
-
-	"github.com/syncthing/syncthing/lib/sha256"
 )
 
 const (

cmd/syncthing/main.go

@@ -92,11 +92,6 @@ above.
  STLOCKTHRESHOLD   Used for debugging internal deadlocks; sets debug
                    sensitivity.  Use only under direction of a developer.
 
- STHASHING         Select the SHA256 hashing package to use. Possible values
-                   are "standard" for the Go standard library implementation,
-                   "minio" for the github.com/minio/sha256-simd implementation,
-                   and blank (the default) for auto detection.
-
  STVERSIONEXTRA    Add extra information to the version string in logs and the
                    version line in the GUI. Can be set to the name of a wrapper
                    or tool controlling syncthing to communicate this to the end

cmd/syncthing/monitor.go

@@ -241,22 +241,6 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
 		if panicFd == nil {
 			dst.Write([]byte(line))
 
-			if strings.Contains(line, "SIGILL") {
-				l.Warnln(`
-*******************************************************************************
-* Crash due to illegal instruction detected. This is most likely due to a CPU *
-* incompatibility with the high performance hashing package. Switching to the *
-* standard hashing package instead. Please report this issue at:              *
-*                                                                             *
-*   https://github.com/syncthing/syncthing/issues                             *
-*                                                                             *
-* Include the details of your CPU.                                            *
-*******************************************************************************
-`)
-				os.Setenv("STHASHING", "standard")
-				return
-			}
-
 			if strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:") {
 				panicFd, err = os.Create(locations.GetTimestamped(locations.PanicLog))
 				if err != nil {