Document removal of ~/.pam_environment support from openssh

I intend to remove support for reading ~/.pam_environment from openssh-server in trixie, since it has a history of security problems. Explain this in the release notes to try to minimize the number of people surprised by this.

Thanks to Richard Lewis for some of the text here, which I adapted, and to Justin B Rye for copy-editing suggestions.

Closes: #1030119