Revert "Start diagnostic group_id at 1 to handle non LS diagnostics (#22694) (#22700)

This reverts commit 3ae6aa0e4d.

If "group_id = 0" really did mean a diagnostic not from a language
server then various methods related to diagnostic set would need to be
updated. Something like [this
diff](https://gist.github.com/mgsloan/e902153bcaec207b39260a8f40d3134d).

Plan instead is to use InfoPopover instead of DiagnosticPopover for
these.

Release Notes:

- N/A

crates/language/src/buffer.rs

@@ -208,10 +208,10 @@ pub struct Diagnostic {
     /// The human-readable message associated with this diagnostic.
     pub message: String,
     /// An id that identifies the group to which this diagnostic belongs.
-    /// 0 is used for diagnostics that do not come from a language server.
     ///
-    /// When a language server produces a diagnostic with one or more associated diagnostics, those
-    /// diagnostics are all assigned a single group ID.
+    /// When a language server produces a diagnostic with
+    /// one or more associated diagnostics, those diagnostics are all
+    /// assigned a single group ID.
     pub group_id: usize,
     /// Whether this diagnostic is the primary diagnostic for its group.
     ///