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.

git-svn-id: https://develop.svn.wordpress.org/trunk@60097 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/upgrade-develop-testing.yml

@@ -64,7 +64,7 @@ jobs:
         db-type: [ 'mysql' ]
         db-version: [ '5.7', '8.4' ]
         # WordPress 4.9 is the oldest version that supports PHP 7.2.
-        wp: [ '4.9', '6.5', '6.7', '6.8' ]
+        wp: [ '4.9', '6.5', '6.7', '6.8-RC1' ]
         multisite: [ false, true ]
 
         exclude:

.github/workflows/upgrade-testing.yml

@@ -66,7 +66,7 @@ jobs:
         php: [ '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4' ]
         db-type: [ 'mysql' ]
         db-version: [ '5.7', '8.0', '8.4', '9.1' ]
-        wp: [ '6.7', '6.8' ]
+        wp: [ '6.6', '6.7' ]
         multisite: [ false, true ]
 
         exclude:
@@ -101,7 +101,7 @@ jobs:
         php: [ '7.2', '7.4', '8.0', '8.4' ]
         db-type: [ 'mysql' ]
         db-version: [ '5.7', '8.4' ]
-        wp: [ '6.0', '6.1', '6.2', '6.3', '6.4', '6.5', '6.6' ]
+        wp: [ '6.0', '6.1', '6.2', '6.3', '6.4', '6.5' ]
         multisite: [ false, true ]
 
         exclude: