Remove the old release scripts

[skinny85]

In #15331, we introduced a new tool, cdk-release, responsible for performing the version bump, generating the Changelog, etc. However, we still left the old logic intact (accessible by setting the LEGACY_BUMP environment variable), in case the new tool had some bugs.

Now that we have done a few releases using the new tool, it seems to be battle-tested enough, and we're ready to remove the old release process:

1. Remove the code in the LEGACY_BUMP branch of the bump.js script. We should probably change the bump.sh script to call the cdk-release tool directly, and remove the bump.js script completely.
2. We can remove this workaround.
3. We can remove this workaround (it's on the v2-main branch).
4. We can remove the dependency on the standard-version package.

[github-actions]

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