adding comments for GH Actions CI workflow

The latest `setup-beam` GH Action (`v1.16.0` as of 18/10/2023) states that only versions up to OTP 25 are supported for ubuntu-22.04 runners (see [this page](https://github.com/marketplace/actions/setup-erlang-otp-with-optional-elixir-and-mix-and-or-rebar3?version=v1.16.0#compatibility-between-operating-system-and-erlangotp) for more details).

However, for all Linux runners, the `setup-beam` GH Action uses prebuilt OTP releases stored at [builds.hex.pm](https://builds.hex.pm). Those builds are prepared by [Bob the builder](https://github.com/hexpm/bob#erlang-builds), and the actual list of the prebuilt OTP versions for ubuntu-22.04 runners can be found [here](https://github.com/hexpm/bob#erlang-builds).

For more details, check the implementation of [getOTPVersions()](https://github.com/erlef/setup-beam/blob/v1.16.0/dist/index.js#L10120) and [installOTP()](https://github.com/erlef/setup-beam/blob/v1.16.0/dist/index.js#L9721) functions of `setup-beam` GH Action.

.github/workflows/ci.yml

@@ -24,6 +24,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        ## the full list of supported (prebuilt) OTP versions for ubuntu-22.04 runners
+        ## can be found here:
+        ##     https://builds.hex.pm/builds/otp/ubuntu-22.04/builds.txt
         otp: [ '25.3.2.6', '26.1.2' ]
     runs-on: ubuntu-22.04
     env: