bump minimum required ruby to 2.7

As of oauth2 v2.0 the officially supported Ruby versions are 2.7+.
Ruby 2.7 was released over 4 years ago and just recntly entered EOL,
so it's a reasonable cutoff to make (esp when taking into account
oauth2's supported Ruby versions).

The CI suite was already testing against Ruby >= 2.7.0 so there
should be no changes required there.

fitbit_api.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['*.{md,txt}', 'lib/**/*']
   spec.require_paths = ['lib']
 
-  spec.required_ruby_version = '>= 1.9.3'
+  spec.required_ruby_version = '>= 2.7.0'
 
   spec.add_runtime_dependency     'oauth2', '~> 2.0'