Update devise gem to 4.7.3 for Ruby 2.7 deprecation fixes

What does this MR do?

Upgrades devise to fix Ruby 2.7 deprecation warning. Related with #257438 (closed)

devise changelog

4.7.3 - 2020-09-20

• bug fixes
  • Do not modify :except option given to #serializable_hash. (by dpep)
  • Fix thor deprecation when running the devise generator. (by deivid-rodriguez)
  • Fix hanging tests for streaming controllers using Devise. (by afn)

4.7.2 - 2020-06-10

• enhancements

  • Increase default stretches to 12 (by sergey-alekseev)
  • Ruby 2.7 support (kwarg warnings removed)

• bug fixes

  • Generate scoped views with proper scoped errors partial (by shobhitic)
  • Allow to set scoped already_authenticated error messages (by gurgelrenan)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team