[Approved] Plasma 6 proposal: change the release schedule

I'd like to discuss the Plasma release schedule, because it presents a few issues:

1. Shipping three big Plasma releases a year doesn't generally leave enough time for adequate QA and bugfixing before release, contributing to .0 releases being buggy. It interacts awkwardly with the release schedules of several big discrete-release distros such as Kubuntu and Fedora, causing them to frequently make distro releases that have a version of Plasma that's 6+ months old and already out of support from the KDE perspective. It also often causes the last month or two of some Plasma development cycles to coincide with the holiday season and Akademy which isn't ideal because developers' attentions are split. Finally, it stresses the Promo team's resources, so they often can't spend enough time polishing each individual release announcement.
2. Our beta periods are quite short and the beta isn't updated after its initial release, so beta testers frequently report already-fixed issues, wasting everyone's time.
3. Coming up with 3 new wallpapers a year--one per Plasma feature release--has been extremely challenging for VDG.

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To address these issues, I would like to propose the following changes for Plasma 6:

We go down to two feature releases per year, and we adjust the schedule to release them in early March and early September.

With that extra time, we lengthen each feature release's beta period to 6 weeks and update the beta every week.

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My hope for these changes would be more stability, less user confusion, better alignment with distro release schedules, and better use of the Promo team's resources.