Validate ProtectedEnvironments::DeployAccessLevel on the model level

What does this MR do and why?

Adds proper model-level validation to ProtectedEnvironments::DeployAccessLevel.

See more in Data integrity issue on `protected_environment_... (#330483 - closed) and related discussion in !110782 (comment 1308802104)

• Once this is merged, we need to re-run the migration from Cleanup invalid deploy access levels for protec... (!110686 - merged) - I'll open a separate MR.
• After the migration is re-run, we'll the database constraint from Add validity check to deploy access levels (!110782 - closed)

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.

Related to #330483 (closed)