This commit centralizes how feature flags are handled. All feature flags
must now add an entry in the `featureFlagConfig` dictionary. This
dictionary associates the flag with an environment variable name and
optionally a minimum version for CodeQL.
The new logic is:
- if the environment variable is set to false: disabled
- if the minimum version requirement specified and met: disabled
- if the environment variable is set to true: enable
- Otherwise check feature flag enablement from the server
As suggested in review: The `GITHUB_REPOSITORY` environment variable is
only available on Actions. Passing it in explicitly avoids potentially
crashing if this code is called from the runner.