Pending RELEASE-NOTES for the upcoming release

This is work in progress and will change before the release goes public on 15th of September 2021.

Changes:

Bugfixes:

• configure: tweak nghttp2 library name fix
• CURLMOPT_TIMERFUNCTION.3: remove misplaced "time"
• docs/MQTT: update state of username/password support
• Get rid of the unused HAVE_SIG_ATOMIC_T et. al.
• http_proxy: clear 'sending' when the outgoing request is sent

Contributors:

Bastian Krause, Christian Weisgerber, Dan Fandrich, Daniel Stenberg, Josh Soref, Oleg Pudeyev, Ray Satiro, zloi-user on github,