Change rust language server to rust-analyzer

Change rust language server to rust-analyzer

rust-analyzer is the officially blessed Language Server for Rust:
https://github.com/rust-lang/rfcs/pull/2912

Also drop the special support code for RLS.

* eglot.el (eglot-server-programs): Add rust-mode language server
"rust-analyzer" and prefer it to the older "rls".
(eglot-rls, jsonrpc-connection-ready-p)
(eglot-handle-notification): Delete special support for "rls".

* eglot-tests.el (rls-analyzer-watches-files)
(rls-analyzer-hover-after-edit): Rename to ...
(rust-analyzer-watches-files)
(rust-analyzer-hover-after-edit): ... this.  Update tests to work
with rust-analyzer.
* README.md: Update references for RLS to point to rust-analyzer.
* NEWS.md: Announce above change.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/803