refactor(runner): ability to opt-out of using --ephemeral / opt-in to legacy --once for GHES older than 3.3 (#1384)

* runner: Remove the ability to use the deprecated `--once` flag

Ref #1196

* runner: Ability to opt-out of using --ephemeral

Although we are going to eventually remove the ability to use the legacy --once flag as proposed in #1196, there might be folks still using legacy GHES versions 3.2 or earlier.

This commit removes the existing feature flag to opt-in for --ephemeral, while adding another feature flag RUNNER_FEATURE_FLAG_ONCE to opt-in for --once so that folks stuck in legacy GHES versions
can still use ARC.

Since this change every user starts using --ephemeral by default. If they see any issues on legacy GHES instance, RUNNER_FEATURE_FLAG_ONCE=true can be set to opt-in to keep using --once, which gives one more ARC release until they upgrade their GHES instance.

But beware, we won't support legacy GHES instances forever as it's going to be a maintenance nightmare. Please upgrade!

Ref #1196

README.md

@@ -1311,12 +1311,11 @@ spec:
         # Disables automatic runner updates
         - name: DISABLE_RUNNER_UPDATE
           value: "true"
-        # Configure runner with --ephemeral instead of --once flag
+        # Configure runner with legacy --once instead of --ephemeral flag
         # WARNING | THIS ENV VAR IS DEPRECATED AND WILL BE REMOVED
-        # IN A FUTURE VERSION OF ARC. IN 0.22.0 ARC SETS --ephemeral VIA 
-        # THE CONTROLLER SETTING THIS ENV VAR ON POD CREATION.
+        # IN A FUTURE VERSION OF ARC.
         # THIS ENV VAR WILL BE REMOVED, SEE ISSUE #1196 FOR DETAILS
-        - name: RUNNER_FEATURE_FLAG_EPHEMERAL
+        - name: RUNNER_FEATURE_FLAG_ONCE
           value: "true"
 ```