Deprecate remote_auth_plugin option - plugins can use entry points no…

…w. (pantsbuild#16691)

Follow up for pantsbuild#16212

[ci skip-rust]

src/python/pants/option/global_options.py

@@ -1378,6 +1378,18 @@ class BootstrapOptions:
     remote_auth_plugin = StrOption(
         default=None,
         advanced=True,
+        deprecation_start_version="2.15.0.dev2",
+        removal_version="2.16.0.dev1",
+        removal_hint=softwrap(
+            """
+            Remote auth plugins should now provide the function by implementing an entry point called remote_auth.
+
+            If you are developing a plugin, switch to using an entry point.
+
+            If you are only consuming a plugin from someone else, you can delete the remote_auth_plugin option
+            and now only need the plugin to be included in [GLOBAL].plugins
+            """
+        ),
         help=softwrap(
             """
             Path to a plugin to dynamically configure remote caching and execution options.