Encrypt feature flags client tokens

What does this MR do?

This MR changes feature flags client token to encrypted: :optional. Unencrypted tokens will be removed in subsequent release.

There are migrations to:

• Change existing token column to nit null so that we can stop writing to it.
• Add new token_encrypted column
• Add new index by project_id and token_encrypted.
• Post deployment migration to encrypt existing tokens.

Migrations are ported from (as per https://gitlab.com/gitlab-org/gitlab-ee/issues/11252).

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/57918.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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