ci: remove minor releases from the matrix

No point in testing minor Emacs releases, because they have no major
features anyway, fixes only. E.g. given Emacs 25: 25.1, 25.2 and 25.3
are only different in amount of fixes. It isn't expected that
something may break between these releases.

So there's not much point in running tests on all of them. This commit
removes most minor Emacs versions and only leaves in the initial
release for each Emacs major version.

It was brought up here before
#33 (comment)
so far to no opposition.

Author: Hi-Angel

.github/workflows/ci.yml

@@ -13,13 +13,8 @@ jobs:
       matrix:
         emacs_version:
           - 25.1
-          - 25.2
-          - 25.3
           - 26.1
-          - 26.2
-          - 26.3
           - 27.1
-          - 27.2
           - 28.1
           - snapshot
     steps: