fix(deps): update go to 1.21

[ramonpetgrave64]

Fixing the existing PR https://github.com/slsa-framework/slsa-verifier/pull/498 to also change the github actions to use the go 1.21 sourced directly from go.mod.

• https://github.com/slsa-framework/slsa-github-generator/blob/07e64b653f10a80b6510f4568f685f8b7b9ea830/.github/workflows/builder_go_slsa3.yml#L56
• https://github.com/actions/setup-go?tab=readme-ov-file#getting-go-version-from-the-gomod-file
• https://github.com/slsa-framework/slsa-verifier/actions/runs/7559933600/job/20584856777?pr=498

... Error: We were unable to automatically build your code. Please replace the call to the autobuild action with your custom build steps. Encountered a fatal error while running "/opt/hostedtoolcache/CodeQL/2.15.5/x64/codeql/go/tools/autobuild.sh". Exit code was 1 and error was: 2024/01/17 18:06:58 Autobuilder was built with go1.21.5, environment has go1.20.12 ...

Also fixing some more lint checks about repeated strings

[laurentsimon]

Enabling auto-merge