Mail 2.6.1 silences excessive warnings; remove Gemfile hack

Completes https://github.com/rails/rails/pull/15493

Revert "For our build, stick with mail 2.5.x for now"

This reverts commit bea0d10222567d2c930f2a5005d32354b70ac415.
Merge pull request #15575 from bf4/4-0-stable

Mail 2.6.1 silences excessive warnings; remove Gemfile hack