[GDK] Make Ruby 3 the default Ruby for the GDK

Overview

Make Ruby 3.0.x the default Ruby to begin the process of dog fooding 🙂

Proposal

Update .tool-versions to make Ruby 3.0.x the default while keeping 2.7.6 installed.

With the current Ruby 3.x being 3.0.0 and Ruby 2.7.x being 2.7.6 e.g.:

diff --git a/.tool-versions b/.tool-versions
index 110cdefe..c0493122 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -4,7 +4,7 @@
 # GDK for asdf users.
 #
 # <default-version> <other-version>
-ruby 2.7.6 3.0.0
+ruby 3.0.0 2.7.6
 golang 1.15.5
 nodejs 14.15.4
 yarn 1.22.5