Add base64 gem dependency for Ruby 3.4 compatibility

[casperisfine]

I wanted to submit a PR but was a bit surprised with the gemspec and wasn't sure how to add a dependency in it.

So just a quick heads up that Ruby 3.4 will make base64 a "bundled gems", meaning gems now need to declare their dependency on it: https://bugs.ruby-lang.org/issues/20187

[dentarg]

Trying to simplify the gemspec in #681

Since 9b6e3e9 the required Ruby version for ruby-saml is effectively Ruby 2.6.