Add deprecation warning for `treat_symbols_as_metadata_keys_with_true_values`.

Add a custom message to notify users that this is now the default behavior.

I wanted to add a spec that tested the message but when I was looking at the [corresponding spec](https://github.com/rspec/rspec-core/blob/master/spec/rspec/core/configuration_spec.rb#L1003-L1008), I could not figure out what was going on with `expect_deprecation_with_call_site(__FILE__, __LINE__ + 1)`. Let me know how I can update the spec to verify that the custom message is being shown.

For https://github.com/rspec/rspec-core/issues/1166.