Move all "master" uses to "main"

The canonical repository https://github.com/moodlehq/moodle-local_moodlecheck
has moved its "master" branch to "main".

So, here, we are adjusting all the places where the, now old, "master"
branch was being used. Usually that includes:

- CI Integrations (GHA, Travis...).
- Docs: READMEs, change logs, comments, any link pointing to "master" in general.
- Cases in code base that may depend of git branch name (unlikely, but...)

.github/workflows/ci.yml

@@ -28,12 +28,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Highest php versions supported by each branch (with master always being tested twice).
+          # Highest php versions supported by each branch (with main always being tested twice).
           - php: 8.2
-            moodle-branch: master
+            moodle-branch: main
             database: pgsql
           - php: 8.2
-            moodle-branch: master
+            moodle-branch: main
             database: pgsql
           - php: 8.2
             moodle-branch: MOODLE_403_STABLE
@@ -74,12 +74,12 @@ jobs:
             database: pgsql
             plugin-ci: ^3
 
-          # Lowest php versions supported by each branch (with master always being tested twice).
+          # Lowest php versions supported by each branch (with main always being tested twice).
           - php: 8.1
-            moodle-branch: master
+            moodle-branch: main
             database: pgsql
           - php: 8.1
-            moodle-branch: master
+            moodle-branch: main
             database: pgsql
           - php: 8.0
             moodle-branch: MOODLE_403_STABLE