Generate letsencrypt private key on the fly

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/issues/62452

This MR:

• remove lets_encrypt_private_key generation from migration
• brings back generation of this secret on the fly
• addresses situation when this code is executed on secondary GEO node: let's encrypt settings will be disabled there

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

Closes #62452 (closed)