[wp-trac] [WordPress Trac] #59004: Update the `test-old-branches.yml` workflow to include 6.3

[WordPress Trac]

#59004: Update the `test-old-branches.yml` workflow to include 6.3
------------------------------+-------------------------
 Reporter:  davidbaumwald     |      Owner:  (none)
     Type:  enhancement       |     Status:  new
 Priority:  normal            |  Milestone:  6.4
Component:  Build/Test Tools  |    Version:  trunk
 Severity:  normal            |   Keywords:  needs-patch
  Focuses:                    |
------------------------------+-------------------------
 This should've been done when we branched at RC(see
 https://make.wordpress.org/core/handbook/about/release-cycle/releasing-
 major-versions/#branching-before-release), but this was missed.  Chatted
 with @desrosj, and this workflow runs from `trunk` so no need to backport
 immediately before the 6.3 release.

 Props @desrosj.

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Ticket URL: <https://core.trac.wordpress.org/ticket/59004>
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