Drop base64 gem from dependency

[tricknotes]

This following warning is shown in test.

/Users/ryunosuke.sato/src/github.com/lautis/uglifier/spec/spec_helper.rb:3: warning: base64 was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.

Instead of adding base64 to dependency, the usage of Base64#strict_encode64 and Base64#strict_decode64 have replaced with Array#pack and String#unpack1.

Note: Similar PRs are here

• Drop depenency on base64 httprb/http#778
• [Fix #12310] Drop base64 gem from runtime dependency rubocop/rubocop#12313
• Remove base64 dependency rack/rack#2110
• Drop runtime dependency on base64 elastic/elasticsearch-ruby#2295
• Drop base64 gem dependency socketry/protocol-http#51

[lautis]

String#unpack1 was added in Ruby 2.4, so we'd need to bump the required ruby version to 2.4. Given hat support ended in 2019, this is OK to me.