Add manual release workflow

Why is this change being made?

Makes it possible to create RubyGem releases without an upstream pipeline trigger.

We need to create a RubyGem release for the currently supported schema range (https://gitlab.com/gitlab-org/ruby/gems/gitlab-security_report_schemas/-/issues/12). We also need to be able to release different versions of the gem for the same schema range.

This MR:

• moves release-related jobs to the deploy stage.
• adds the manual-release job which triggers the existing release procedure by pushing a git tag.
• changes the published git tags (!30 (diffs, comment 1989560022))
• fixes hard-coded versions in unit tests which otherwise break own-version bumps.
• fixes the integrity_check job (!30 (diffs, comment 1989568058)).

Validation

Tested with my fork and --dry-running rake tasks.

Schema version range change

• added versions to supported_versions in bauerdominic/gitlab-security_report_schemas@06f7ecfa
• release commit updates versions correctly bauerdominic/gitlab-security_report_schemas@5acefe45
• release job creates the correct gem version https://gitlab.com/bauerdominic/gitlab-security_report_schemas/-/jobs/7295017394

Version bump

• minor version bumped bauerdominic/gitlab-security_report_schemas@b7d6632a
• release commit updates versions correctly bauerdominic/gitlab-security_report_schemas@efb3b0eb
• release job creates the correct gem version https://gitlab.com/bauerdominic/gitlab-security_report_schemas/-/jobs/7295541077

Related to https://gitlab.com/gitlab-org/ruby/gems/gitlab-security_report_schemas/-/issues/12, gitlab-org/gitlab#383507, gitlab-org/gitlab#383516