Update CI Matrix to Include Ruby 3.4 and Replace FakeWeb with WebMock

[taketo1113]

This Pull Request introduces the following improvements:

• Update the CI matrix to add Ruby versions 3.0 - 3.4
• Replace fakeweb with webmock for Ruby 3.2 compatibility
• Fix circular require warning on Ruby 3.2 and later
• Fix test for ping http timeout on Ruby 3.1 and later

Details

CI Matrix Update

Adds support for Ruby 3.4, 3.3, 3.2, 3.1, 3.0, and retains 2.7 in the GitHub Actions test matrix.

Replacing FakeWeb with WebMock

The fakeweb gem is no longer maintained and is not compatible with Ruby 3.2 or later.
webmock is an actively maintained and feature-rich alternative for HTTP request stubbing.
https://github.com/bblimke/webmock

This update:

• Modifies README.md and net-ping.gemspec to reflect the dependency change
• Refactors test cases to use WebMock syntax while ensuring backward compatibility

Fix circular require warning

The warning was occurring on Ruby 3.2 and later due to a circular require issue.
This was resolved by removing an unnecessary require statement from net-ping.gemspec.

/Users/taketo/.rbenv/versions/3.2.4/bin/ruby -w -I"lib" /path/net-ping/tmp/ruby/3.2.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb "test/test_net_ping_external.rb" 
<internal:/Users/taketo/.rbenv/versions/3.2.4/lib/ruby/3.2.0/rubygems/core_ext/kernel_require.rb>:86: warning: <internal:/Users/taketo/.rbenv/versions/3.2.4/lib/ruby/3.2.0/rubygems/core_ext/kernel_require.rb>:86: warning: loading in progress, circular require considered harmful - /Users/taketo/.rbenv/versions/3.2.4/lib/ruby/3.2.0/rubygems.rb
	from <internal:gem_prelude>:2:in  `<internal:gem_prelude>'