Update dependabot to use groups except for nullaway and errorprone

Nullaway and errorprone must be updated in lockstep to avoid
other issues with compatibility, so are not suitable for
being changed as part of a wider group. If ErrorProne updates,
we have to ensure NullAway has made a fix first otherwise the
build shall fail.

Author: ascopes

.github/dependabot.yml

@@ -11,6 +11,11 @@ updates:
       - ascopes
     labels:
       - ci
+    groups:
+      actions:
+        patterns:
+          - "*"
+
   - package-ecosystem: maven
     directory: /
     open-pull-requests-limit: 99
@@ -22,3 +27,12 @@ updates:
       - ascopes
     labels:
       - maven
+    groups:
+      maven:
+        patterns:
+          - "*"
+        exclude-patterns:
+          # Excluded from grouping to generate individual PRs:
+          # Error Prone and NullAway versions must be kept in lockstep.
+          - "com.google.errorprone:*"
+          - "com.uber.nullaway:*"