Fix PHP 8.1 "implicit conversion from float to int" exception on certain avatar colors which blocks rendering user pages

Summary:
Since PHP 8.1, conversion from float to integer should be explicit.

https://wiki.php.net/rfc/implicit-float-int-deprecate

According to phlog, the alpha channel value of the automatically created user
avatar image for a new user account sometimes is a float.

Thus always round() the alpha channel value to be an integer.

Closes T15375

Test Plan: Applied this change, created five user accounts via http://phorge.localhost/people/new/standard/ , and each resulting alpha value in PhabricatorFilesComposeAvatarBuiltinFile.php was an integer according to phlog (see corresponding Maniphest task). Rendering of each new user page always succeeded.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15375

Differential Revision: https://we.phorge.it/D25209