Use the --ephemeral flag by default

[toast-gear]

Context
The flag --ephemeral has been the standard flag for a while and we should migrate to using it by default now that GHES 3.3 has been out for over 3 months.

Currently, a runner is configured with the --once flag unless the runner has the environment variable RUNNER_FEATURE_FLAG_EPHEMERAL: "true" set. This env var should be changed to be ENABLE_LEGACY_EPHEMERAL: "true" which results in the runner being configured with the --once flag instead of the default --ephemeral flag.

The bulk of the work has been done on PR #831, it just needs updating to use the desired env var name and the docs need updating:

1. the new flag needs adding in https://github.com/actions-runner-controller/actions-runner-controller#runner-entrypoint-features
2. The Ephemeral Runner section needs turning into Static Runners as by default our runners are ephemeral. The configuration option is ephemeral: false and so that's what needs documenting.

EDIT edited to reflect ethmoson's comment

[ethomson]

The Ephemeral Runner section needs turning into Long-Lived Runners (or something like that) as by default our runners are ephemeral. The configuration option is ephemeral: false and so that's what needs documenting.

The Actions team uses the term static runners as the opposing notion to ephemeral runners.

[toast-gear]

OK new-ish plan. We are going to remove the --once flag entirely in the subsequent release to this issue being released (due to be part of the next milestone to be released).

Tasks:

• Update controller to set RUNNER_FEATURE_FLAG_EPHEMERAL to true as part of a runner creation
• Update docs to highlight this change as well as highlight --once is being removed
• Update runner entrypoint to highlight --once is deprecated