Text style options in RTE don't support Heading 5 and 6

Summary

Rich-text mode Text style drop down only supports up to Heading 4. Heading 5 and Heading 6 are supported if I type something like ##### HeadingText. If I have my cursor on a Heading 5 or Heading 6, the Text Style drop-down is greyed out and can't be interacted with.

Proposal

Add Heading 5 and 6 to the dropdown so that the dropdown can be interacted with.