Branching tasks for 6.8

[desrosj]

This is a ticket for tracking the changes required immediately before and after creating the 6.8 branch.

These are ​detailed in the Core Handbook.

Changes to commit to the 6.8 branch after branching:

• Pinning highest supported version of PHP for local environment.
• Changing local workflow references to remote ones using npm run grunt replace:workflow-references-local-to-remote.

Trac ticket: https://core.trac.wordpress.org/ticket/63164

Trac ticket: https://core.trac.wordpress.org/ticket/63164

[desrosj]

In 60093:

Post 6.8 branching version update.

Trunk is now 6.9-alpha

Fixes #63164.

[desrosj]

Seems [60092] did not make it's way to this ticket. Linking for future reference.

[desrosj]

In 60097:

Build/Test Tools: Adjust how the 6.8 branch is included in matrices.

Because 6.8 is not yet officially released, 6.8 cannot be used in a test matrix.

This removes 6.8 from the Upgrade Testing workflow and changes the value used for the Upgrade Develop Version Testing workflow to 6.8-RC1 to confirm that updating from 6.8-RC1 to trunk (which is now 6.9-alpha) is successful.

Follow-up to [60093].

Props mukesh27.
See #63164.

6.8 has not yet officially been released, so it's unavailable for testing.

Trac ticket: https://core.trac.wordpress.org/ticket/63164.

Merged in https://core.trac.wordpress.org/changeset/60097.