chore: Remove deprecated scripts after helm-charts workflow migration

## Context

The helm-charts repository has completed migrating from inline bash scripts to reusable composite actions from this repo. The following scripts are now deprecated and can be removed after successful E2E testing:

### Scripts to Remove

- `.github/scripts/attestation-lib.sh` - Functions replaced by:
- `attestation/generate-digest`
- `attestation/create`
- `attestation/extract-map`
- `attestation/update-pr-map`

- `.github/scripts/version-bump.sh` - Functions replaced by:
- `version-bump/determine-bump`
- `version-bump/calculate-version`
- `version-bump/update-manifest`
- `changelog/generate`
- `changelog/update`

### Acceptance Criteria

- [ ] E2E tests pass in helm-charts with actions-only workflows
- [ ] Remove `attestation-lib.sh` from helm-charts
- [ ] Remove `version-bump.sh` from helm-charts
- [ ] Update any documentation referencing these scripts

### Related

- helm-charts workflow migration: Phases 1-6 complete
- Migration plan: `.claude/plans/workflow-migration/`