Remove `ssh_banned_key` feature flag

What does this MR do and why?

Describe in detail what your merge request does and why.

Removes the ssh_banned_key feature flag, as the feature is ready to be fully released.

• Feature issue: #24614 (closed)
• Rollout issue: #363410 (closed)

How to set up and validate locally

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

1. Go to $GDK_URL/-/profile/keys

2. Copy a public key from https://github.com/rapid7/ssh-badkeys/tree/master/authorized, or use this one

   ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAwRIdDlHaIqZXND/l1vFT7ue3rc/DvXh2yx5EFtuxGQRHVxGMazDhV4vj5ANGXDQwUYI0iZh6aOVrDy8I/y9/y+YDGCvsnqrDbuPDjW26s2bBXWgUPiC93T3TA6L2KOxhVcl7mljEOIYACRHPpJNYVGhinCxDUH9LxMrdNXgP5Ok= mateidu@localhost

3. Paste the key in the text field and click Add key

4. Receive error indicating that the key is compromised

   

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.