Add support for Active Admin 3.0

[alexplatteeuw]

Active Admin has introduced a major release which requires to use Ransack v4 due to security considerations.

For more context:

• https://github.com/activerecord-hackery/ransack/issues/1273
• https://younes.codes/posts/how-to-hack-with-ransack
• https://github.com/activeadmin/activeadmin/pull/8009

[tf]

Added appraisal to also run tests against Active Admin 3 and rebased to current based.

[tf]

Released as 1.8. Thank you for your contribution!