Repair wp/7.0 branch Actions: Pin wp-env's WordPress Core version

[desrosj]

What?

Pins wp-env's WordPress core checkout to WordPress/WordPress#7.0-branch via .wp-env.json's "core" value, and removes the wordpress test matrix dimension, the WP_ENV_CORE environment variable override, and the compute-previous-wordpress-version job it depended on from unit-test.yml's test-php job.

Why?

.wp-env.json's "core" value was unpinned ("WordPress/WordPress"), so every PHP unit test run built its WordPress environment against whatever commit happened to be on WordPress Core's own trunk at run time. That makes test-php non-reproducible and ties this frozen branch's CI to unrelated, ongoing changes on a different project's default branch — the opposite of what a maintenance branch's CI should depend on. Pinning to Core's own 7.0-branch tests against the WordPress version this Gutenberg branch is actually meant to run alongside, and removing the "previous major version" matrix leg (along with the job that computed which version that was) drops a comparison that only made sense while tracking Core's moving trunk.

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.

[github-actions]

Flaky tests detected in 324a3d7.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/desrosj/gutenberg/actions/runs/34634343874
📝 Reported tests:

should undo typing after non input change in /test/e2e/specs/editor/various/undo.spec.js, passed after 1 failed attempt.

Error: expect(received).toEqual(expected) // deep equality

- Expected  - 2
+ Received  + 2

  Object {
    "blockIndex": 0,
-   "endOffset": 16,
-   "startOffset": 16,
+   "endOffset": 4,
+   "startOffset": 4,
  }

Call Log:
- Timeout 5000ms exceeded while waiting on the predicate
    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/various/undo.spec.js:122:3

[github-actions]

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Co-authored-by: desrosj <desrosj@git.wordpress.org>

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[desrosj]

Migrated to real upstream: WordPress#82802. Closing this fork PR.