Update TR history.

Please merge to 6.1.
Start filling in RacketCon titles and abstracts.
Commented out some test cases that intentionally failed, so as to not
get spurious failure messages.
heap-remove! sometimes needs to heapify up

closes PR 14651
misc improvements around read-language (uses and docs)

- make a logger for drracket-related #lang problems
- change read-language's docs to indicate that it can return #f
- adjust a bunch of calls to read-language for the case that it returns #f
  with the goal of making that be the same as a get-info function that
  always returns the given default
- mention `read-language` in the guide section entitled
 "Source-Handling Configuration"
More titles and abstracts for RacketCon.
redex: add #:pre to define-judgment-form

also, update contract error messages for the same
redex: fix error messages for #:satisfying

Closes PR 14642
Add `flatten-all-begins` to syntax/flatten-begin
Add doc examples for `flatten-begin`
remove call to 'time'

Please add to 6.1 release branch
fix bug in ->i
fix a test for `raco exe`

The test failed for certain installation configurations due to a
problem with the test.
raco exe: document command-line flags
sync: accept 0 arguments

As suggested by Jonathan Schuster.

Note that the `choice-evt` constructor already accepted 0 arguments.
racket/draw: fix `get-argb-pixels` for unscaled and just-alpha

Closes PR 14653

Merge to v6.1
racket/draw: fix problems with `set-argb-pixels` on scaled, no-alpha bitmaps

Also, fix the docs to clarify that `just-alpha?` as #t means a no-op for
a target bitmap that has an alpha channel.
web pages: add "recent" links at download-racket-lang.org

These links reflect soft links that are in place at
"mirror.racket-lang.org", but they're needed as links/rewrite on the
main site for some purposes, such as references from the tech-report
pages.
web pages: use UTC for computing month names

Otherwise, generated web pages can differ depending on which timezone
is used to render them.
web pages: fix platform sorting on download pages

Fix use of wrong array element during sort, where using the wrong element
caused "Windows source" to be sorted before "Windows (x86, 32-bit)".
(The wrong-element bug was introduced with changed to support the button
that downloads directly from the default mirror.)

Closes PR 14655 and PR 14656