chore: stop Dependabot proposing major jest-dom bumps (#10280)

@testing-library/jest-dom 7 declares "engines": {"node": ">=22"}, whilst Node
20 remains our minimum and is what most of the buildfarm runs. The bump passes
our JS tests on Node 20 in practice, so this is a deliberate choice not to
depend on an officially unsupported combination rather than a reaction to a
failure.

The "^6.9.1" constraint in web/package.json already prevents 7.x from being
installed; what Dependabot proposes is widening that constraint, which is the
part we do not want, so ignore major updates for this package until the
buildfarm moves to Node 22. See #10271 and #10210.

.github/dependabot.yml

@@ -108,3 +108,9 @@ updates:
         update-types:
           - "minor"
           - "patch"
+    # @testing-library/jest-dom 7 requires Node >= 22, whilst Node 20 is the
+    # minimum we support and what most of the buildfarm runs. Revisit once the
+    # buildfarm moves to Node 22. See #10271 and #10210.
+    ignore:
+      - dependency-name: "@testing-library/jest-dom"
+        update-types: ["version-update:semver-major"]