Enable ci_skip_before_parsing_yaml by default

What does this MR do?

Enables the ci_skip_before_parsing_yaml feature flag by default, which moves the processing of skipped pipelines before the validation of .gitlab-ci.yml, in order to save processing time by not parsing the YAML for skipped pipelines.

The feature flag has been ON on gitlab.com since Monday (August 9).

Related issues

Feature flag rollout issue: #337167 (closed)
Feature issue: #235855 (closed)