Make granting of open-source license to public projects configurable

What does this MR do and why?

As part of https://gitlab.com/gitlab-org/gitlab/-/issues/351565, we want to stop granting open-source licenses to public projects. The MR introduces a new legacy_open_source_license_available feature flag to be able to switch the behaviour at a later point.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/351565

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Output of DB migrations

$ rake db:migrate:up VERSION=20220203133652                                                                                                                                                                     
== 20220203133652 AddLegacyOpenSourceLicenseAvailableToProjectSettings: migrating 
-- add_column(:project_settings, :legacy_open_source_license_available, :boolean, {:default=>true, :null=>false})
   -> 0.0083s
== 20220203133652 AddLegacyOpenSourceLicenseAvailableToProjectSettings: migrated (0.0083s) 

$ rake db:migrate:down VERSION=20220203133652                                                                                                                                                                   
== 20220203133652 AddLegacyOpenSourceLicenseAvailableToProjectSettings: reverting 
-- remove_column(:project_settings, :legacy_open_source_license_available, :boolean, {:default=>true, :null=>false})
   -> 0.0030s
== 20220203133652 AddLegacyOpenSourceLicenseAvailableToProjectSettings: reverted (0.0041s)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.