Pin specific Gutenberg plugin version for E2E tests in 6.5 branch

[desrosj]

Gutenberg 19.8 will [ ​https://github.com/WordPress/gutenberg/pull/67117raise the minimum required version of WordPress from 6.5 to 6.6].

In [59220], a gutenberg-version input was added to the reusable E2E testing workflow to account for these situations. [59221] made use of this new input for the 6.4 branch when the minimum was last bumped.

Version 19.7 will be the last version of the plugin that supports WP 6.5. The last bug fix release for that version should be pinned in the matrix of that workflow for this branch when 19.8 enters pre-release state.

​https://github.com/WordPress/gutenberg/pull/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 ​https://github.com/WordPress/gutenberg/commit/58e2d4b7ef9df24c537b1a2cf9485c2b2849822d).

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

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

[desrosj]

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

Thank _you_ for merging, @desrosj! 😄