addressing pull request review comments

- added back rhel9 to the cvmfsexec distribution creation logic since a distribution for el9 can be generated on an el9 machine

creation/create_cvmfsexec_distros.sh

@@ -25,7 +25,7 @@ or a comma-separated list of values from the options {osg|egi|default}.
 
 PLATFORMS_LIST (optional): indicates machine types (platform- and architecture-based)
 for which distributions is to be built. Can be empty, a single value or a
-comma-separated list of values from the options {rhel7-x86_64|rhel8-x86_64|suse15-x86_64|rhel8-aarch64|rhel8-ppc64le}.
+comma-separated list of values from the options {rhel9-x86_64|rhel7-x86_64|rhel8-x86_64|suse15-x86_64|rhel8-aarch64|rhel8-ppc64le}.
 EOF
 }
 
@@ -182,7 +182,7 @@ fi
 # after confirming that the remaining number of arguments to be either 1 or 2
 re_sources="^((osg|egi|default),)*(osg|egi|default),?$"
 # TODO: update the regex for rhel9 and other machine types upon checking with `makedist -h` periodically
-re_mtypes="^((rhel(7|8)|suse15)(-(x86_64|aarch64|ppc64le),?))+$"
+re_mtypes="^((rhel(7|8|9)|suse15)(-(x86_64|aarch64|ppc64le),?))+$"
 # check whether the first argument is sources (strict ordering followed)
 if ! [[ "$1" =~ $re_sources ]]; then
  error_handler "Invalid source(s) provided (comma-separated list with osg|egi|default), not '$1'"
@@ -194,7 +194,7 @@ elif [[ "$2" =~ $re_mtypes ]]; then
  machine_types=$2
 else
  # handle if there were typos in the arguments passed
- error_handler "Invalid platform provided. Must be empty or comma-separated list with (rhel7-x86_64|rhel8-x86_64|suse15-x86_64|rhel8-aarch64|rhel8-ppc64le), not '$2'"
+ error_handler "Invalid platform provided. Must be empty or comma-separated list with (rhel7-x86_64|rhel8-x86_64|suse15-x86_64|rhel9-x86_64|rhel8-aarch64|rhel8-ppc64le), not '$2'"
 fi
 
 echo "(Re)Building of cvmfsexec distributions enabled!"