Migrate from master to main branch

Use of the `master` and `slave` terms in the computer industry is
widespread, but has been recently discouraged, as it was brought to
attention that the terms may be considered offensive.
GitHub has already moved their default repositories from the `master`
to `main` in many places. We're planning to do the same for our
repositories, this however requires some changes in the code.
After the proposed changes get merged to the `master` branch, the
repository should be ready for the migration from `master` to `main`.

Author: michalinacienciala

.github/workflows/client.yml

@@ -3,7 +3,7 @@ name: Go
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
   workflow_dispatch: