Chore: Move Gutenberg_REST_Templates_Controller from 6.2 to 6.3 compatibility.

[jorgefilipecosta]

The changes in Gutenberg_REST_Templates_Controller_6_3 will not be part of the 6.2 release so this PR just moves them to the 6.3 compatibility folder.

[github-actions]

Flaky tests detected in 5719f70.
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/WordPress/gutenberg/actions/runs/4177076975
📝 Reported issues:

• [Flaky Test] should always expand single line selection #39787 in specs/editor/various/multi-block-selection.test.js
• [Flaky Test] should be able to create an hierarchical post without title support #36939 in specs/editor/plugins/custom-post-types.test.js
• [Flaky Test] should close the inserter when clicking on the toggle button #48005 in /test/e2e/specs/editor/various/inserting-blocks.spec.js
• [Flaky Test] should leave a scheduled page scheduled if canceled #47499 in specs/editor/various/switch-to-draft.test.js
• [Flaky Test] should revert a scheduled page to a draft if confirmed #47500 in specs/editor/various/switch-to-draft.test.js