Update Travis to run at 9.0 and 9.1 with deprecations

The main changes are

-    # Allow this many deprecation warnings.
-    - SYMFONY_DEPRECATIONS_HELPER=1
+    # Initialise the real SYMFONY_DEPRECATIONS_HELPER variable.
+    - SYMFONY_DEPRECATIONS_HELPER=0
+    # Create a default for the allowed deprecations per branch.
+    - DEPRECATIONS=0

and

+    - php: 7.4
+      env:
+        - DRUPAL_CORE=9.1.x
+        # 66 Declaring ::setUp without a void return typehint
+        - DEPRECATIONS=66

and

+  # Get the allowed number of deprecation warnings.
+  - SYMFONY_DEPRECATIONS_HELPER=$DEPRECATIONS || $SYMFONY_DEPRECATIONS_HELPER
+  - echo $SYMFONY_DEPRECATIONS_HELPER

This is a clearer way to specify the number of expected deprecations per branch. Any more than this number will cause the test to fail and hence alert us (me) of the problem.

Also the build is currently broken at 9.0 due to additionally installing composer in the module directory travis_retry composer install -d $TESTDIR. This is not needed, causes problems at 9.0+ and is better removed.

The rest of the changes are just updating comments for clarity and relevance.

Thanks, this is helpful to me, and hence my contribution to the project.