Soft deprecated Ruby 2.1 and 2.2

`fog-core` is only actively testing Ruby 3.0+ as part of the CI process
resulting in upstream dependencies like Excon including gems extracted
from the stdlib without constrained versions using Ruby language
features unsupported by Ruby <2.3 versions.

This means we are soft removing 2.1 and 2.2 support so whilst we are not
actively declaring a higher version is required, thing will fail unless
you declare your application to use older versions.

.github/workflows/ruby.yml

@@ -37,7 +37,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ["2.1", "2.2", "2.3"]
+        ruby-version: ["2.3"]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby

README.md

@@ -44,6 +44,12 @@ and its modules:
 As required by the main `fog-core` library, support for Ruby 1.9 was dropped in
 `v1.0.0` of this gem.
 
+Upstream changes on dependencies have resulted in support failing for Ruby <2.3
+from `v1.12.0` due to some libraries failing to declare usage of newer language
+features in their own gemspec files.
+
+As of 2024 `fog-core` is only testing for Ruby 3.0+ support.
+
 ## Contributing
 
 1. Fork it ( https://github.com/fog/fog-brightbox/fork )