chore(ci): remove packageManager injection workaround from version bump workflow

## Context

PRs #10952 and #10972 added a workaround to `.github/workflows/release-version-bump.yaml` that injects `packageManager` into `package.json` when absent. This was needed because older `core/*` branches (e.g. `core/1.42`) predate the `packageManager` field added in #10687, and `pnpm/action-setup` needs either `version` input or `packageManager` to resolve pnpm.

## Action

Once all active `core/*` branches have `packageManager` in their `package.json`, remove the "Ensure packageManager field exists" step from the workflow. It will be a no-op by then.

**Do not action before May 8, 2026** — give time for all release branches to roll forward past #10687.

## Steps

1. Verify all active `core/*` branches have `packageManager` in `package.json`
2. Remove the `Ensure packageManager field exists` step from `release-version-bump.yaml`
3. Test a version bump on both `main` and a `core/*` branch

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10975-chore-ci-remove-packageManager-injection-workaround-from-version-bump-workflow-33c6d73d36508112aebafbf6c7d527f6) by [Unito](https://www.unito.io)