bug#61478: 28.2; eglot keeps sending `didSave` notifications despite the active server not supporting `textDocumentSync`

[~kby]

I've come across this bug whilst using the Dart language server, which is included in the Dart SDK.

Steps to reproduce bug:

- Create example Dart project using Flutter: `flutter create hello_world`

- Open `hello_world/lib/main.dart` in Emacs

- Start eglot with `M-x eglot RET`

- Make a rudimentary change in the file; ex. add a line break

- Save with `C-x C-s`

- Following text appears in the minibuffer area: `[eglot] Server reports (type=1): Unknown method textDocument/didSave`

Related bug reports (on GitHub): https://github.com/dart-lang/sdk/issues/36464 and https://github.com/microsoft/language-server-protocol/issues/288

I am using eglot 1.11 (from ELPA) with Emacs 28.2 on Arch.

    Thanks in advance.