build: Use correct range specification for Go version

The old `^1.20.7` means `1.x.y, >= 1.20.7` which allows 1.21.0, which
was not intended. The new `~1.20.7` means `1.20.x, >= 1.20.7`, which is
safer.

.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: "~1.20.7"
 
   # Optimize compatibility on the slow archictures.
   GO386: softfloat