Support Ruby 3.0

[andyundso]

This PR adds support for Ruby 3.0. It needed a couple of changes, including a few that are technically for Ruby 2.5 support.

• Delete the extra minitest entry in the Gemfile. As explained in the commit, the gemspec locked minitest to v5.14.0, but the version from the Gemfile had higher priority, which meant it installed 5.17.0 which isn't compatible anymore with Ruby 2.5.
• Lock bundler version on CI to 2.3.16, as this is the last release that supports Ruby 2.5.
• I tried to build the Windows native gem by running bundle exec rake gem:windows, but this failed with a Rake error that it is unable to find this task. I updated the Rake task in this project to match the example from the rake-compiler-dock docs, which made the build start.
• I got a very long error message when trying to compile with OpenSSL v1.1.1d, so I updated it to v1.1.1s.
• And at last, compile the ports and the gem itself against Ruby 3.0.

I tested the changes against a project of mine. It's a "traditional" Rails v6.1 application using 6.1.2.1. I ran our CI with a version of the gem build based on my changes and everything is green with the following combinations:

• Ruby 2.7, x64-mingw32, SQL Server Express 2017
• Ruby 3.0, x64-mingw32, SQL Server Express 2017
• Ruby 2.7, x86_64-linux, FreeTDS v1.2.20, SQL Server Express 2017
• Ruby 2.7, x86_64-linux, FreeTDS v1.2.20, SQL Server Express 2019
• Ruby 3.0, x86_64-linux, FreeTDS v1.2.20, SQL Server Express 2017
• Ruby 3.0, x86_64-linux, FreeTDS v1.2.20, SQL Server Express 2019

Credits to @bryanwieg for the initial work in #521.

[bryanwieg]

@andyundso this looks really nice!

@aharpervc will probably be the one to review it.

[aharpervc]

This is cool... very exciting. Are you able to also do any of the suggestions here? At least having build artifacts for this would be great to get more people for testing.

[andreasnef]

Would be great to get this as I assume many (as us) are planning moving to v3 soon.

[orgads]

Please add also 3.1 and 3.2.

[andyundso]

As long as things are still moving here, I created a fork with support for Ruby 3.0, 3.1 and 3.2, including a fat gem for Windows.

[orgads]

Thank you!

[andyundso]

Replaced with #530.