all: Fix various user-facing and non-user-facing typos (#8509)

Found via `codespell -q 3 -S lang,./gui/default/vendor -L benchs,bu,inflight,ro`

cmd/syncthing/cli/index_idxck.go

@@ -57,7 +57,7 @@ func indexCheck(*cli.Context) (err error) {
 	defer func() {
 		if err == nil {
 			if success {
-				fmt.Println("Index check completed succesfully.")
+				fmt.Println("Index check completed successfully.")
 			} else {
 				err = errors.New("Inconsistencies found in the index")
 			}
@@ -349,7 +349,7 @@ func indexCheck(*cli.Context) (err error) {
 
 func needsLocally(vl db.VersionList) bool {
 	gfv, gok := vl.GetGlobal()
-	if !gok { // That's weird, but we hardly need something non-existant
+	if !gok { // That's weird, but we hardly need something non-existent
 		return false
 	}
 	fv, ok := vl.Get(protocol.LocalDeviceID[:])

cmd/syncthing/main.go

@@ -591,7 +591,7 @@ func syncthingMain(options serveOptions) {
 	}
 
 	// Check if auto-upgrades is possible, and if yes, and it's enabled do an initial
-	// upgrade immedately. The auto-upgrade routine can only be started
+	// upgrade immediately. The auto-upgrade routine can only be started
 	// later after App is initialised.
 
 	autoUpgradePossible := autoUpgradePossible(options)

cmd/syncthing/monitor.go

@@ -273,7 +273,7 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
 * This crash usually occurs due to one of the following reasons:                *
 *  - Syncthing being stopped abruptly (killed/loss of power)                    *
 *  - Bad hardware (memory/disk issues)                                          *
-*  - Software that affects disk writes (SSD caching software and simillar)      *
+*  - Software that affects disk writes (SSD caching software and similar)       *
 *                                                                               *
 * Please see the following URL for instructions on how to recover:              *
 *   https://docs.syncthing.net/users/faq.html#my-syncthing-database-is-corrupt  *