Proposed schedule for JavaFX 21

[Kevin Rushforth]

Here is the proposed schedule for JavaFX 21.

RDP1: Jul 13, 2023 (aka “feature freeze”)
RDP2: Aug 3, 2023
Freeze: Aug 31, 2023
GA: Sep 19, 2023

We plan to fork a jfx21 stabilization branch at RDP1.

The start of RDP1, the start of RDP2, and the code freeze will be 16:00 
UTC on the respective dates.

Please let Johan or me know if you have any questions.

-- Kevin