* 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