Fix parser dependency

[kxmbrian]

Why

If you're on Ruby 3 and version 3.x of the parsergem, when you upgrade the scout_apm gem to 5.3.6, you will no longer be able to bundle install. Sample error:

Could not find compatible versions

Because scout_apm >= 5.3.6 depends on parser ~> 2.6
  and Gemfile depends on scout_apm = 5.3.6,
  parser ~> 2.6 is required.
So, because Gemfile depends on parser ~> 3.3.0,
  version solving has failed.

It looks like gemspecs are not evaluated dynamically on your local machine, but when the gem is published:

> gem dependency scout_apm -v 5.3.6 --remote
Gem scout_apm-5.3.6
  activerecord (>= 0, development)
  activesupport (>= 0, development)
  addressable (>= 0, development)
  guard (>= 0, development)
  guard-minitest (>= 0, development)
  m (>= 0, development)
  minitest (>= 0, development)
  mocha (>= 0, development)
  parser (~> 2.6)
  pry (>= 0, development)
  rake-compiler (>= 0, development)
  rubocop (>= 0, development)
  simplecov (>= 0, development)
  sqlite3 (>= 0, development)

@jrothrock @natematykiewicz

What

Reverses this change in #486.

I'm not sure if we should be putting a warning somewhere in the scout_apm gem, but the parser gem gives a warning like this (not sure about older versions), which might suffice:

> rails server
...
warning: parser/current is loading parser/ruby32, which recognizes 3.2.3-compliant syntax, but you are running 3.2.2.
Please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

[lancetarn]

Uff da. I've yanked 5.3.6. We'll get this cleaned up and cut another tag today.

[natematykiewicz]

Oh no. Never yank! Anyone who has that version installed is going to have problems with deployments now.

You need to cut a release ASAP.