Fix editor_session_start event custom property key

[dcalhoun]

Prior to this change, wpios_editor_session_start events were not sent to Tracks, as they were marked as invalid with the following error. This relates to changes made in #16848 and #16832.

Custom properties dictionary keys must contain alpha characters and underscores only.

To test:

1. Install 18.2 beta build.
2. Launch post editor.
3. ⚠️ Verify wpios_editor_session_start event does not arrive to Tracks.
4. Install build from this PR.
5. Launch post editor.
6. ✅ Verify wpios_editor_session_start event arrives to Tracks with expected properties.

Regression Notes

1. Potential unintended areas of impact
   Custom properties for the event are missing.
2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Verified custom properties arrived with the event in Tracks.
3. What automated tests I added (or what prevented me from doing so)
   None. We would need to either write an integration test to verify the session event property keys are "valid" with Automattic-Tracks-iOS or iterate over them manually asserting their validity.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading it from AppCenter here with build number: 56210. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[chipsnyder]

LGTM 👍 Thanks for fixing this @dcalhoun

[chipsnyder]

cc @antonis

[dcalhoun]

Hey @mokagio. 👋🏻 This is my first time targeting a beta release. Is it alright to merge this regression fix into release/18.2 now? Are there any other steps I need to complete? Thanks! 🙇🏻

[mokagio]

Hey @dcalhoun!

This is my first time targeting a beta release

😄 👏

Sorry about missing this ping yesterday. I did see the PR in the list, but just assumed it was waiting to be merged by the author 😞

Anyways... Yes the only thing you need to do is merge it. The release train conductor (currently me) will take it from there and generate a new beta build.

We don't automatically generate new betas when a merge on the branch happens because there are times when multiple PRs get merged in a single day and we don't want to overload the comms channel with consecutive betas.

[mokagio]

I'm going to merge this myself now to make up for letting it sit here for so long 😳

[mokagio]

@dcalhoun this has been bundled as part of 18.2 beta 1 (18.2.0.1).

Thanks for your work 🙌