all: Use Go 1.21, new QUIC API (#9040)

.github/workflows/build-infra-dockers.yaml

@@ -6,7 +6,7 @@ on:
       - infrastructure
 
 env:
-    GO_VERSION: "^1.20.5"
+    GO_VERSION: "^1.21.0"
     CGO_ENABLED: "0"
     BUILD_USER: docker
     BUILD_HOST: github.syncthing.net

.github/workflows/build-syncthing.yaml

@@ -12,7 +12,7 @@ env:
   # The go version to use for builds. We set check-latest to true when
   # installing, so we get the latest patch version that matches the
   # expression.
-  GO_VERSION: "~1.20.7"
+  GO_VERSION: "~1.21.0"
 
   # Optimize compatibility on the slow archictures.
   GO386: softfloat
@@ -48,7 +48,7 @@ jobs:
         runner: ["windows-latest", "ubuntu-latest", "macos-latest"]
         # The oldest version in this list should match what we have in our go.mod.
         # Variables don't seem to be supported here, or we could have done something nice.
-        go: ["1.19", "1.20"]
+        go: ["1.20", "1.21"]
     runs-on: ${{ matrix.runner }}
     steps:
       - name: Set git to use LF