docs: add deprecation to communicate ropc discouragement

[james-d-elliott]

This adds godoc deprecations to the compose.OAuth2ResourceOwnerPasswordCredentialsFactory and oauth2.ResourceOwnerPasswordCredentialsGrantHandler in order to clearly communicate the discouragement of the ROPC grant type to users implementing this library.

Related Issue or Design Document

#650 (comment)

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

Further comments

No testing is applicable to this change as far as I am aware.

[aeneasr]

Thank you so much!