devel/ruby-activemodel61: update to 6.1.5.1

## Rails 6.1.5.1 (April 26, 2022) ##

*  No changes.

## Rails 6.1.5 (March 09, 2022) ##

*  Clear secure password cache if password is set to `nil`

    Before:

      user.password = 'something'
      user.password = nil

      user.password # => 'something'

    Now:

      user.password = 'something'
      user.password = nil

      user.password # => nil

    *Markus Doits*

*  Fix delegation in `ActiveModel::Type::Registry#lookup` and `ActiveModel::Type.lookup`

    Passing a last positional argument `{}` would be incorrectly considered as keyword argument.

    *Benoit Daloze*

*  Fix `to_json` after `changes_applied` for `ActiveModel::Dirty` object.

    *Ryuta Kamizono*

(taca)