Deprecate warn/2, add warning/2

[altjohndev]

For library changes, see CHANGELOG.

Upgraded CI Elixir from 1.10.4 OTP 23.0.4 to 1.16.1 OTP 26.2.1.

Upgraded dev dependencies:

• :credo from 1.1 to 1.7 (1.1 breaks on Elixir 1.16.1)
• :dialyxir from 1.1.0 to 1.4 (1.1.0 breaks on Elixir 1.16.1)
• :ex_doc from 0.25 to 0.31 (to sync with the updated design and features)

This should be a minor release (from 1.6.1 to 1.7.0) since it is adding MetaLogger.warning/2 and deprecating MetaLogger.warn/2.