update to go1.21.6

go1.21.6 (released 2024-01-09) includes fixes to the compiler, the runtime, and
the crypto/tls, maps, and runtime/pprof packages. See the Go 1.21.6 milestone on
our issue tracker for details:

- https://github.com/golang/go/issues?q=milestone%3AGo1.21.6+label%3ACherryPickApproved
- full diff: https://github.com/golang/go/compare/go1.21.5...go1.21.6

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/test.yml

@@ -63,7 +63,7 @@ jobs:
         name: Set up Go
         uses: actions/setup-go@v5
         with:
-          go-version: 1.21.5
+          go-version: 1.21.6
       -
         name: Test
         run: |