Add a missing base64 require

[mdkent]

We observed this today during a routine bin/kamal envify -d staging:

Acquiring the deploy lock...
  ERROR (NameError): Exception while executing on host foo-staging-app-01: uninitialized constant Kamal::Commands::Lock::Base64
/Users/mkent/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/kamal-1.3.0/lib/kamal/commands/lock.rb:26:in `write_lock_details'
/Users/mkent/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/kamal-1.3.0/lib/kamal/commands/lock.rb:8:in `acquire'
/Users/mkent/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/kamal-1.3.0/lib/kamal/cli/base.rb:109:in `block (2 levels) in acquire_lock'
/Users/mkent/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/sshkit-1.21.7/lib/sshkit/backends/abstract.rb:31:in `instance_exec'
/Users/mkent/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/sshkit-1.21.7/lib/sshkit/backends/abstract.rb:31:in `run'
/Users/mkent/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/sshkit-1.21.7/lib/sshkit/runners/parallel.rb:12:in `block (2 levels) in execute'

I'm guessing one of the underlying gems we bundle installed stopped requiring base64, but I haven't tracked down which one.

While we're at it, this also prepares for the moving of base64 from default to a bundled gem in ruby 3.4 (see rails/rails#48907).

[mdkent]

Oof, before merging this it looks like gemfiles/rails_edge.gemfile can't run with the older ruby version.

And #630 should be merged first as well.

/cc @djmb

[mdkent]

Aha, tracked this down. Looks like we used to get base64 via net-ssh, but they dropped it net-ssh/net-ssh@3120304

I can reproduce this issue on a fresh install of ruby.