feat: Remove unused upgrade test from Makefile

Remove the upgrade-tests option from Makefile.
This test was used for the upgrade path from RHEL7 to RHEL8 but was not hooked in CI. Today we use a newer version of the test in the common repository, and it is unnecessary to have it here.
This option does not work anyway since the run_upgrade_test was significantly changed, and this option was probably forgotten in the Makefile.

Author: Lupphes

Makefile

@@ -14,7 +14,3 @@ include common/common.mk
 clean-hook: clean-versions
 
 script_env += NOT_RELEASED_VERSIONS="$(NOT_RELEASED_VERSIONS)"
-
-# Additional upgrade tests.  Not hooked into CI ATM.
-upgrade-tests: $(VERSIONS)
-	OS=$(OS) test/run_upgrade_test 9.2:remote 9.4:local 9.5:local 9.6:local 10:local