📖 Bump version tag in examples

[pnacht]

The generic generator's README still used @v1.2.0, this PR bumps all references to the current @v1.2.1.

[laurentsimon]

This is a recurring problem with docs being out of sync. Are you interested in helping us:

1. Polishing up the RELEASE.md process so that docs is updated
2. Add an automated pre-release test to verify the refs are set properly? We do already have some pre-release tests but not for docs (https://github.com/slsa-framework/slsa-github-generator/blob/main/.github/workflows/release.yml#L19)

Created #1188 for tracking

[laurentsimon]

Thanks!