Execute Jetpack::init only once

[szepeviktor]

Already executed in line number 86.

Fixes #19166
Fixes #5684

All is explained in the above issue.

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

• No

Testing instructions:

• Try using different parts of Jetpack depending on init.
• Try updating from the stable version of Jetpack to a new version with this patch, and ensure that the upgrade routine works well.

[jeherve]

I've closed the issue you opened since we've discussed this in the past in #5684. See the links there for more details about the things we've done with this in the past.

Since this is an important part of Jetpack, could you add some testing instructions to your PR, as well as a changelog entry?

You will also need to add a changelog entry, as explained here, so the tests can pass:
https://github.com/Automattic/jetpack/blob/master/docs/writing-a-good-changelog-entry.md
https://github.com/Automattic/jetpack/blob/master/docs/monorepo.md#using-the-jetpack-changelogger

Thank you!

[szepeviktor]

Since this is an important part of Jetpack, could you add some testing instructions to your PR, as well as a changelog entry?

Changelog entry added.
I have zero clue what test could help here.

[jeherve]

I've tested this, it seems to work well. When updating, the upgrade routine still runs well, and no issues came up after that.