Build/Test Tools: Remove npx commands in 6.5

[adimoldovan]

Backport of r63309 to the 6.5 branch, already in trunk via #13021. It replaces the two remaining npx calls in Gruntfile.js with npm exec --no, which fails on a missing package instead of installing it, and declares update-browserslist-db as a direct devDependency.

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

Testing instructions

1. Run npm ci.
2. Run npm exec --no -- update-browserslist-db --version. It prints browserslist-lint 1.0.13 and downloads nothing.
3. Run npm exec --no -- wp-scripts. It prints Script name is missing., which confirms the local binary ran.
4. Move node_modules/update-browserslist-db out of the tree, then run npm exec --no -- update-browserslist-db again. 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 run grunt -- --help. Confirm that wp-packages:update and browserslist:update still appear in the task list.

Use of AI Tools

AI assistance: Yes — Claude Code (Claude Opus 5) applied r63309 to the 6.5 branch and verified the changeset.

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