Build/Test Tools: Pin Gutenberg version to 19.7.0

[ockham]

WordPress/gutenberg#67117 bumped the Gutenberg plugin's minimum required WordPress version to 6.6. That change was first shipped as part of GB 19.8.0 (see WordPress/gutenberg@58e2d4b).

This means that GB 19.7.0 (i.e. the last release from the 19.7.x series) was the last version that supported WordPress 6.5.

As a consequence, the end-to-end tests on Core's 6.5 branch need to be updated to pin the Gutenberg version to that version, so that E2E test will use that version (rather than a newer one) for testing Core's compatibility with the Gutenberg plugin.

See https://core.trac.wordpress.org/changeset/59221 for the analogue change for the 6.4 branch.

Trac ticket: https://core.trac.wordpress.org/ticket/62488

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[desrosj]

Thanks for following up, @ockham!

[desrosj]

Merged in https://core.trac.wordpress.org/changeset/59518.

[ockham]

Thank you for merging, @desrosj! 😄