Set PHP version to 8.1

** Why are these changes being introduced:

* PHP 8.0 is currently only receiving maintenance updates, and we want
  to understand whether our application and plugins will function
  correctly on a future version of PHP.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/pw-11

** How does this address that need:

* This updates the version of PHP used to 8.1, which is still under
  active development by the community.

** Document any side effects to this change:

* I guess we will see. We have many questions about what might break if
  we do this upgrade.

pantheon.upstream.yml

@@ -2,7 +2,7 @@ api_version: 1
 web_docroot: true
 # See https://pantheon.io/docs/pantheon-yml/#enforce-https--hsts for valid values.
 enforce_https: transitional
-php_version: 8.0
+php_version: 8.1
 database:
   version: 10.4
 build_step: true