fix: Add a warning about updating the on attribute before upgrade

[castastrophe]

What has changed and why

Summarize files edited as part of this MR along with a brief description of what was changed/why.

• Added a warning to be thrown before upgrade if the tag contains the on attribute
• Updated documentation and tests, removed current examples of manually pushing the on attribute inside the pfe-content-set

Testing instructions

Be sure to include detailed instructions on how your update can be tested by another developer.

1. Check out the branch.
2. Edit the pfe-content-set demo page to add the on="light" attribute on the pfe-content-set and pfe-cta tags. Add an ID to one of the tags.
3. Run npm install && npm run start pfe-accordion.
4. Preview the accordion's demo page.
5. Check that the console is registering 2 warnings and that the one for the tag with an ID lists the ID in the warning in square brackets:

pfe-content-set[#foo]: The "on" attribute is protected and should not be manually added to a component. The base class will manage this value for you on upgrade.

pfe-cta: The "on" attribute is protected and should not be manually added to a component. The base class will manage this value for you on upgrade.

Ready-for-merge Checklist

• Expected files: all files in this pull request are related to one feature request or issue (no stragglers)?
• Did you update or add any necessary documentation (README.md, WHY.md, etc.)?
• Was this feature demo'd and the design review approved?
• Did you update the CHANGELOG.md file with a summary of this update?

Be sure to share your updates with the patternfly-elements-contribute@redhat.com mailing list!

[starryeyez024]

👍 for testing and documentation, still needs JS review

much awesomeness:

[castastrophe]

@kylebuch8 Update applied! Working great!

[castastrophe]

@kylebuch8 I updated the storybook instances to show theme support.

[castastrophe]

@kylebuch8 Reverted the storybook updates and moved them into a separate branch. Prettier is still showing updates to the .storybook files but I can't undo that at this point.

[kylebuch8]

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