Update release notes for 1.14

Pinging @larkost and really everyone else. This is almost entirely from the closed tickets on the 1.14 milestone excluding bugs that seemed like they never made it into releases, but there are likely things missed and/or things that should be excluded. I tried to mention the bits of backward incompatibility in the "Compatibility" block.
I believe by 'next version' it means when going from 1.14 to 1.15 or higher, but the wording is a little ambiguous.