Add support for rails 8.0.0.alpha

[zunair-dev]

I'm facing this on rails db:migrate

PaperTrail 15.1.0 is not compatible with ActiveRecord 8.0.0.alpha. We allow PT
contributors to install incompatible versions of ActiveRecord, and this
warning can be silenced with an environment variable, but this is a bad
idea for normal use. Please install a compatible version of ActiveRecord
instead (>= 6.1, < 7.2). Please see the discussion in paper_trail/compatibility.rb
for details.

as obviously it supports rails <7.2

[navidemad]

The paper-trail-gem maintainers only release new versions once they is a new rails stable—no betas or alphas.
For now, you can set the environment variable to suppress the warning.
However, use this with caution, as the gem might still require adjustments:

PT_SILENCE_AR_COMPAT_WARNING=1 

[zunair-dev]

alrigh @navidemad, I'll suppress the warning until we get support for rails 8.