chore: release the three packages as one unit, starting with a beta

[derrickmehaffy]

Replaces #200 (closed). Three problems with how that PR would have released.

1. It would have created three releases, and two would have failed

Each package was declared separately, so merging produced three tags and three
draft GitHub Releases:

plugin-rest-cache-v5.1.0
provider-rest-cache-memory-v5.1.0
provider-rest-cache-redis-v5.1.0

Publishing any one fires publish.yml, which publishes all three packages.
So the first would have done the entire release, and the second and third would
have failed with cannot publish over the previously published versions: 5.1.0
— two red runs immediately after a successful release.

2. The changelog was split three ways

Three <details> sections, no single place describing the release.

3. Fixed: one release, one changelog

These packages are versioned in lockstep and published together, so they are one
releasable unit. The root is now the only release — one tag, one GitHub release,
one CHANGELOG.md. The three package.json versions are updated through
extra-files, which is what keeps them in step; workspace: + pnpm pack
already handles the ranges between them.

Two things fall out of it:

• The PR title interpolates ${version} again. plugins/merge.js takes it
  from the release at path ., and there wasn't one — which is why it read a
  bare chore: release.
• The tag is v<version>, matching every tag this repo has used since v4,
  and satisfying the v* tag rule on the npm-latest / npm-prerelease
  environments.

4. Beta first

versioning: prerelease + prerelease-type: beta, so the next release is
5.1.0-beta, published to the next dist-tag via the npm-prerelease
environment.

To graduate: drop prerelease, prerelease-type and versioning from the
package and the following release is a stable 5.1.0 on latest. A second beta
increments rather than colliding — bumpPrerelease bumps a trailing number.

Verified by dry run against this branch

✔ updating from 1.0.0 to 5.1.0-beta
Would open 1 pull requests
updates: packages/plugin-rest-cache/package.json
         packages/provider-rest-cache-memory/package.json
         packages/provider-rest-cache-redis/package.json
         CHANGELOG.md
         .release-please-manifest.json

Note prerelease: true alone does nothing — the version stayed 5.1.0 until
versioning: prerelease was added. Both are needed.

Known: duplicated changelog entries

Several entries appear twice with different shas:

* add a cache statistics endpoint for the admin dashboard (594ba4a)
* add a cache statistics endpoint for the admin dashboard (56aeae4)

That is real history — 594ba4a is the merge commit for #171 and 56aeae4 is
the commit on its branch. release-please walks all 82 commits since v5.0.0
rather than the 47 on first-parent, and both parse to the same conventional
commit. It is confined to the old merge-commit era; PRs are squashed now, so it
will not recur. Worth hand-editing out of the release notes on the draft before
publishing, since this one release sweeps up a year of history.

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