ci: Update the branch and tag exclusions on AppVeyor CI

As we plan to release libpng-1.8.0, with the 'libpng18' branch being
the new default, we are also discontinuing the 'master' branch as an
alias branch. Update the exclusions in the AppVeyor CI configuration
file, replacing 'libpng00' to 'libpng18' with 'master'.

Also update the regular expression that excludes release tags to
account for all 'vNN.NN.NN' tag names.

Author: ctruta

.appveyor.yml

@@ -2,8 +2,8 @@ version: 1.6.x-{build}
 
 branches:
   except:
-    - /libpng[0-1][0-8]/
-    - /v[0-1][.][0-8][.][0-9]+/
+    - /master/
+    - /v[0-9]+[.][0-9]+[.][0-9]+/
 
 image:
   - Visual Studio 2022