Build/Test Tools: Remove npx commands in 6.9

[adimoldovan]

Replaces the two remaining npx calls in Gruntfile.js with npm exec --no, and adds update-browserslist-db as a direct dev dependency. Backport of r63309, already in trunk via #13021, to the 6.9 branch.

Trac ticket: https://core.trac.wordpress.org/ticket/65864

Testing instructions

No workflow calls either grunt task, so CI does not exercise this change. Check it locally:

1. Run npm ci.
2. Run npm exec --no -- update-browserslist-db --help. It prints the CLI help and downloads nothing.
3. Run npm exec --no -- wp-scripts. It prints its own Script name is missing. usage error, which shows the local binary resolved.
4. Delete node_modules/update-browserslist-db and node_modules/.bin/update-browserslist-db, then run npm exec --no -- update-browserslist-db. It fails with npx canceled due to missing packages and no YES option instead of fetching the package. Restore the tree with npm ci.
5. Run npm exec --no -- grunt --help. Both wp-packages:update and browserslist:update are still listed.

Use of AI Tools

AI assistance: Yes — Claude Code (Claude Opus 5) applied r63309 to the 6.9 branch and verified the changeset.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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