(chore): Bump Go version to 1.21 + regenerate testdata

[everettraven]

Description of the change:

• Bumps Go version in go.mod to 1.21
• Updates GitHub actions to pull Go version from go.mod file
• Regenerates testdata

Motivation for the change:

• Update to latest Go version to pick up any fixes
• fixes Regenerate testdata #6603

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[grokspawn]

Do we need to adjust content of documentation, for e.g.

• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/upgrading-sdk-version/v1.25.0.md
• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/installation/_index.md#prerequisites-1
• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/building-operators/golang/installation.md#additional-prerequisites
• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/contribution-guidelines/developer-guide.md#prerequisites

?

[everettraven]

• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/upgrading-sdk-version/v1.25.0.md

Not this one. This is a migration guide to v1.25.0 of the SDK. Should go unchanged to allow for following upgrades sequentially.

• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/installation/_index.md#prerequisites-1

Should be done here: https://github.com/operator-framework/operator-sdk/pull/6604/files#diff-67f7d617c6c3b9946e5191cddc06275ceb839d85039812a2b44a22d18e1497b7R91

• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/building-operators/golang/installation.md#additional-prerequisites

Should be done here: https://github.com/operator-framework/operator-sdk/pull/6604/files#diff-ed4ae522b378054b142772bee33ceab89dc2f99d39a40717ed75a1f09c39c3b7R14

• https://github.com/operator-framework/operator-sdk/blob/master/website/content/en/docs/contribution-guidelines/developer-guide.md#prerequisites

Looks like I missed this one. Will update.

[rashmigottipati]

/lgtm