Allow public projects API to modify container_registry_access_level

What does this MR do?

We are in the process of migrating code to use ProjectFeature#container_registry_access_level instead of Project#container_registry_enabled.

Current state:
We have migrated to use ProjectFeature#container_registry_access_level, but container_registry_access_level can currently hold 2 out of 3 possible values: ProjectFeature::ENABLED and ProjectFeature::DISABLED. This mimics the behavior of Project#container_registry_enabled.

This is because container_registry_access_level can currently only be set by using the container_registry_enabled boolean attribute in the public API. Setting container_registry_enabled to true sets container_registry_access_level to ProjectFeature::ENABLED and setting container_registry_enabled to false sets container_registry_access_level to ProjectFeature::DISABLED.

This MR:

1. Allows project_features.container_registry_access_level to be modified by the public projects API. This will allow users to set container_registry_access_level to ProjectFeature::PRIVATE.
2. Deprecates the container_registry_enabled attribute in the public projects API, in favor of container_registry_access_level.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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

Related to #18792 (closed)