tests: Update test matrix

There's no 2.5.x branch any more. Change the default to master, i.e. the
latest release, now that the branch actually reflects the release and
no longer includes patches accidentally merged to the wrong branch.

Keep v2.5.1 around just for safety as it probably is the most widely
used release around.

Would be nice if the 2.6.0 branch was named dev or devel and wasn't
about to disappear as soon as 2.6.0 is released... :-/

.github/workflows/tests.yaml

@@ -10,7 +10,6 @@ jobs:
           - _defaults: # see Dockerfile for variable defaults
           - TASK_VERSION: v2.4.4
           - TASK_VERSION: v2.5.1
-          - TASK_VERSION: 2.5.3
           - TASK_VERSION: 2.6.0
           - VIM_VERSION: v7.4.1546
           - VIM_VERSION: v8.0.0027

Dockerfile

@@ -1,6 +1,6 @@
 ARG ALPINE_VERSION=3.12
 ARG PYTHON_VERSION=3.9
-ARG TASK_VERSION=v2.5.2
+ARG TASK_VERSION=master
 ARG VIM_VERSION=v8.2.0716
 ARG VIMWIKI_VERSION=master