Update release notes for 1.64

[pietroalbini]

This PR makes a few updates to the release notes to bring them on-par with the release notes of other releases:

• The wording on some items has been changed to be more consistent and better describe what each change means, rather than using the PR title.
• Items that were also in the compatibility notes have been removed from the other sections, only keeping them in the compat notes.
• Diagnostics and ICE fixes have been removed from the notes.
• The cargo section has been made more consistent.

r? @Mark-Simulacrum cc @rust-lang/release @joshtriplett

[Mark-Simulacrum]

@bors r+ rollup

[bors]

:pushpin: Commit b4937c3c7bc37baf3e603c9434972b9a5251abb8 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[pietroalbini]

Whelp pushed a commit to the wrong branch.

@bors r=Mark-Simulacrum rollup

[bors]

:pushpin: Commit b4937c3c7bc37baf3e603c9434972b9a5251abb8 has been approved by Mark-Simulacrum

It is now in the queue for this repository.