DO NOT auto-merge up from 3 to 4

[GuySartorelli]

This module has the merge up CI - but we DO NOT want to merge up from 3 to 4.

Acceptance criteria

• Linkfield merge up do not run for the 3 to 4 branch.
• There's a document process for disabling merge ups on specific branches.
• Merge ups between 4 and 5 still occur.

PR

• https://github.com/silverstripe/gha-merge-up/pull/32

[GuySartorelli]

PR merged