Fix the OPA version bump workflow to work with the new README

🗣 Description

This PR fixes the OPA version bump workflow by removing the code that assumes the old README format and tries to modify it.

Closes #1187

🧪 Testing

Example PR created from a scheduled workflow run: example OPA Update PR. You can also fork ScubaGear, specifically this branch, to test this PR or your own fork.

📷 Screenshots (if appropriate)

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✅ Pre-approval checklist

[x] This PR has an informative and human-readable title.
[x] PR targets the correct parent branch (e.g., main or release-name) for merge.
[x] Changes are limited to a single goal - eschew scope creep!
[x] Changes are sized such that they do not touch excessive number of files.
[x] All future TODOs are captured in issues, which are referenced in code comments.
[x] These code changes follow the ScubaGear content style guide.
[x] Related issues these changes resolve are linked preferably via closing keywords.
[x] All relevant type-of-change labels added.
[x] All relevant project fields are set.
[x] All relevant repo and/or project documentation updated to reflect these changes.
[x] All automated checks (e.g., linting, static analysis, unit/smoke tests) passed.

✅ Pre-merge checklist

[ ] PR passed smoke test check.
[ ] Feature branch has been rebased against changes from parent branch, as needed

Use Rebase branch button below or use this reference to rebase from the command line.
[ ] Resolved all merge conflicts on branch
[ ] Notified merge coordinator that PR is ready for merge via comment mention

✅ Post-merge checklist

[ ] Feature branch deleted after merge to clean up repository.
[ ] Verified that all checks pass on parent branch (e.g., main or release-name) after merge.

cisagov / ScubaGear

Fix the OPA version bump workflow to work with the new README #1188