Cirq v1.6.1 release

Patch release to fix#7536, a bug incirq.kraus which made it
in some cases return erroneous near-zero matrices. The release
also includes minor documentation updates.

Contributors

Doug Strain, Michael Hucka, Pavol Juhas, Shih-Cheng Tu

Patches included w/r to v1.6.0

c200724 Test all notebooks vs stable Cirq after the release (#7561)
58ab7ee Link to Willow and Weber data sheets (#7560)
7e3bfca Update OpenQASM compatibility table (#7555)
77546b5 Fix typos in reference docs (#7539)
6f1a975 Fix kraus channels for fallbacks to super-operators (#7537)
7fd4a7b Fix two typos (#7533)