Consolidate action release flow around semver labels

[juan-fernandez]

Summary

• replace the datadog-ci-specific release script with a single scripts/release-action.sh flow
• infer the next action release from merged PR labels: semver-patch or semver-minor
• keep --tag as an explicit override and require --allow-version-mismatch when the requested tag is lower than labels imply
• update the datadog-ci bump helper to apply datadog-ci-version-bump plus the right semver label
• add --dry-run to the datadog-ci bump helper
• document the GitHub CLI requirement and add a PR check that enforces semver release labels
• remove unnecessary environment-variable configuration from the release scripts

Flow

To bump the default datadog-ci-version, run scripts/create-datadog-ci-bump-pr.sh --dry-run first, then scripts/create-datadog-ci-bump-pr.sh if the preview looks right. The helper checks the latest DataDog/datadog-ci release, updates action.yaml and README.md, opens a PR, and labels it with datadog-ci-version-bump plus either semver-patch or semver-minor.

To release the action after one or more labeled PRs are merged, run scripts/release-action.sh --dry-run, then scripts/release-action.sh if the preview looks right. The release script reads merged PR labels since the latest immutable action tag, chooses the next action version, moves the major tag, and creates a GitHub Release with GitHub-generated notes.

For non-datadog-ci changes, maintainers only need to label the PR with semver-patch or semver-minor; the same release script handles it. --tag remains available for explicit releases, with --allow-version-mismatch required when overriding to a smaller bump than labels imply.