Update RLS

This update includes the ability to warn on deprecated config keys.
It's important to be able to warn the user whenever they're using
an old configuration rather than giving them a cryptic "unknown
configuration error"

cc https://github.com/rust-lang/rls-vscode/issues/639

Since we removed a config value in the current nightly, it'd be
very good if this change can make also make it before cutting the
next release.

diff --git a/src/tools/rls b/src/tools/rls
index bdfcef769fcdc777320d452ac71afacad48b0de7..7b0a20bf13b7061b1eb31a058117ac5517ff8cc9 160000 (submodule)
--- a/src/tools/rls
+++ b/src/tools/rls
@@ -1 +1 @@
-Subproject commit bdfcef769fcdc777320d452ac71afacad48b0de7
+Subproject commit 7b0a20bf13b7061b1eb31a058117ac5517ff8cc9