Add required_ruby_version to gemspec to require Ruby 3.1.0 or greater

[synthead]

Related to #316.

The gemspec doesn't specify a minimum Ruby version, and this lib uses omitted keyword arguments, which is a feature introduced in Ruby 3.1.0:

• Values in Hash literals and keyword arguments can be omitted. [Feature #14579]

  • {x:, y:} is syntax sugar for {x: x, y: y}.
  • foo(x:, y:) is syntax sugar for foo(x: x, y: y).

This means that it's possible to install the timex_datalink_client gem on a version of Ruby that is too old, i.e. 3.0.6. When attempting to use the gem on 3.0.6, this exception is raised:

<internal:/home/elaine/.rbenv/versions/3.0.6/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require': /home/elaine/repos/timex_datalink_client/lib/timex_datalink_client.rb:127: syntax error, unexpected ',' (SyntaxError)
      serial_device:,
                    ^
	from <internal:/home/elaine/.rbenv/versions/3.0.6/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from examples/protocol_7/clear-data:10:in `<main>'

A minimum version of 3.1.0 should be set by adding this to the gemspec:

s.required_ruby_version = ">= 3.1.0"

This will present a definitive error to the user if they attempt to install this gem on an older version:

timex_datalink_client-0.12.3 requires ruby version >= 3.1.0, which is incompatible with the current version, ruby 3.0.6p216