Masterbar: avoid php warning with PHP 8.2

[jeherve]

Proposed changes:

• Follow-up to PHP Compatibility: fix dynamic property deprecation notices #30786 and Tiled Galleries: fix dynamic property deprecation notices #31296

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

• Primary issue: 27927

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• On a site running PHP 8.2, go to Jetpack > Settings > Writing and enable the WordPress.com toolbar (masterbar module).
• Check your logs; you should not see any warnings.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ All commits were linted before commit.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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Jetpack plugin:

• Next scheduled release: July 5, 2023.
• Scheduled code freeze: June 26, 2023.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack plugin. Once the plugin is active, go to Jetpack > Jetpack Beta > Jetpack and enable the fix/php82-dynamic-properties-masterbar branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/php82-dynamic-properties-masterbar
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[anomiex]

Offtopic: This sort of PR is why semver probably doesn't make sense for Jetpack-the-plugin. Is this random field on this class part of a public API? If so, making it private should be 'major' significance. But chances are it's not supposed to be.

Anyway, I see nothing outside this class using it in the monorepo, or in wpdirectory, or in a8c internal codebases.