Close #4461 - Use rust-analyzer by default

Use rust-analyzer by default instead of rls, as rls has been deprecated.

test/test_filetype_linter_defaults.vader

@@ -90,7 +90,7 @@ Execute(The defaults for the python filetype should be correct):
   AssertEqual [], GetLinterNames('python')
 
 Execute(The defaults for the rust filetype should be correct):
-  AssertEqual ['cargo', 'rls'], GetLinterNames('rust')
+  AssertEqual ['analyzer', 'cargo'], GetLinterNames('rust')
 
   let g:ale_linters_explicit = 1