ci: ignore mocha semver-major updates

Mocha 12 declares engines "^20.19.0 || >=22.12.0" and its CLI entrypoint
uses import attributes (`with { type: "json" }`), so it cannot run on
Node 16 or 18 at all — the syntax is a parse error there.

Both versions are still first-class entries in the CI test matrix
([16.x, 18.x, 20.x, 22.x, 24.x]), and the compiler targets
@tsconfig/node16, so the package advertises support for them. Taking the
major fails those legs outright; dropping them from the matrix instead
would leave the package claiming support for runtimes it never exercises.

This mirrors the existing @types/node rule, which holds the same support
floor for the same reason. Raising that floor is a deliberate decision,
not something a routine dependency bump should force.

Note this must be a config rule rather than an `@dependabot ignore`
comment: those suppress exactly one major each, so they accumulate and a
fresh PR arrives with every subsequent release — precisely the problem
#240 cleaned up for @types/node.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dhensby

.github/dependabot.yml

@@ -57,6 +57,15 @@ updates:
       # is only moved when the support floor is raised.
       - dependency-name: "@types/node"
         update-types: ["version-update:semver-major"]
+      # Mocha 12 declares engines "^20.19.0 || >=22.12.0" and uses import
+      # attributes, so it cannot run on Node 16 or 18 — both of which are still
+      # in the CI test matrix (see also @tsconfig/node16, and the @types/node
+      # rule above, which holds the same floor). Taking the major would fail
+      # those legs, and silently dropping them would leave the package claiming
+      # support for runtimes it never tests. Move this when the support floor is
+      # deliberately raised, not as a routine dependency bump.
+      - dependency-name: "mocha"
+        update-types: ["version-update:semver-major"]
   - package-ecosystem: "github-actions" # Keep workflow action versions up to date
     directory: "/"
     schedule: