Deprecate custom progress bar styles

Maintaining these is not particularly useful, since the progress bar
provided by rich is capable of degrading gracefully to ASCII and
alternative styles provided today aren't particularly usable either.

src/pip/_internal/cli/req_command.py

@@ -276,6 +276,13 @@ def make_requirement_preparer(
                 gone_in="22.1",
             )
 
+        if options.progress_bar not in {"on", "off"}:
+            deprecated(
+                reason="Custom progress bar styles are deprecated",
+                replacement="to use the default progress bar style.",
+                gone_in="22.1",
+            )
+
         return RequirementPreparer(
             build_dir=temp_build_dir_path,
             src_dir=options.src_dir,