CI Secrets: remove feature flags

What does this MR do?

This MR removes couple of feature flags related to CI Secrets and Vault integration:

1. ci_secrets_syntax - this one is global and enables the new secrets keyword in CI yaml
2. ci_secrets_management - this one is per project and when enabled we include the secrets configuration (if any) in the API and pass it to the runner. Named the same way as the licensed feature so we can use it with Project#beta_feature_available?

The feature flags were enabled and tested on GitLab.com. The shared runners fleet is now upgraded to Runner 13.4 RC. Here is the documenattion MR which is ready to be merged - !38552 (merged).

This is the feature flags rollout issue - #243431 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team