Deprecate some component attributes

[jovyntls]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Fixes #1504

Overview of changes:

• Removes the implementation of the following deprecated attributes:

Component	Current attribute name	Deprecated attribute name
Dropdown	header	text
Box	header	heading
Popover	header	title
Modal	header	title

• Using these attributes will no longer raise a warning.
• These attributes are also removed from the documentation.

Anything you'd like to highlight / discuss:
The _warnDeprecatedAttributes function was removed as well.

Also, I notice that the master branch doesn't seem to be passing all tests on npm run test (not as a result of this PR), but this can be fixed with npm run updatetest.
Should commit the files generated by npm run updatetest here as well? The non-updated tests are not a result of this PR so I'm not sure if it should be in the scope of this PR. 😅

Proposed commit message: (wrap lines at 72 characters)

Deprecate attributes for Dropdown, Box, Popover, Modal

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[tlylt]

Also, I notice that the master branch doesn't seem to be passing all tests on npm run test (not as a result of this PR), but this can be fixed with npm run updatetest

I just ran the tests again locally on master and was able to pass all.
Curious have you tried

• npm run setup and then npm run test to ensure that all dependencies are up to date? Else, perhaps a screenshot of the failing test would be great:)
• ensured that the master branch of your fork is in sync
  

[jovyntls]

Thanks @tlylt , that works!

Have updated the issue description :)

[jonahtanjz]

Nice work @jovyntls! A couple of suggestions. Otherwise the rest looks good.

Thanks @tlylt for helping out 👍

[jonahtanjz]

Thanks for making the changes @jovyntls :)

LGTM 👍