Enable gitaly feature flags by default

What does this MR do and why?

As follow up to gitlab#421712 (comment 1517496919) it was noted that it may be preferable to run all E2E tests with Gitaly Feature Flags enabled. This will allow us to identify potential regressions when they are introduced, as at the moment, we won't detect issues until the FF is marked as enabled by default.

See internal/featureflag/featureflag.go#L17-26 for implementation details in gitaly.

Corresponding change for gitlab-org/gitlab: E2E: Add pipeline to run with gitaly feature flags (gitlab!157784)

How to set up and validate locally

• All tests that interact with either Gitaly/Praefect should succeed

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.