[tlbuild] tl21 schedule

[Karl Berry]

Hi TL builders - I posted a tentative schedule for the 2021 software
collection on https://tug.org/texlive a while ago. It's similar to last
year's.

This is the year of Knuth's tune-up. Going by previous tune-ups, I
expect he will release the new versions long before we are ready to
finalize anything. In any case, we will wait for his release if we have to.

Meanwhile, I've updated the TL sources with the latest release of
asymptote (2.68). If you want to give it a whirl now, it couldn't hurt.
It failed for me :( (parser::parseURL not found by linker; I'm following
up with John).

Let me know if problems, as always. --thanks, karl.

Plan for TeX Live 2021:
15feb: candidate/final sources committed, test builds begin.
28feb: tlnet (and TL'20) frozen, pretest starts, CTAN updates continue there.
15mar: code freeze for final build, major bug fixes only.
27mar: final updates from CTAN, final doc tweaks.
29mar: deliver TL image for TeX Collection packaging/testing.
 5apr: deliver TeX Collection DVD image for manufacturing.
10apr: public release (also of MacTeX).
June?: delivery of DVDs to members.