Add deprecation notice displayed on self-hosted sites using the Masterbar module.

[fgiannar]

[fgiannar]

The following diff ensures that on WPCOM we rely on mu-wpcom-plugin instead of the Jetpack plugin. This needs to be deployed first to avoid deprecation Warnings on WPCOM:

D153139-code

[fgiannar]

The following PR ensures that we rely on the jetpack-masterbar package to load the known dashicon list in the admin-menu endpoint:

https://github.com/Automattic/jetpack/pull/38025

[fgiannar]

Marking as blocked till the above PR and diff are merged

[fgiannar]

D153559-code was deployed covering deploying but then reverting D153139-code