tendermint-rs dropping v0.24.x

[thanethomson]

As per tendermint/tendermint#9155, Tendermint Core is no longer going to support v0.35 or v0.36, and so tendermint-rs will be dropping support for its v0.24.x pre-release series that targeted v0.35 compatibility.

A new branch main has been created from the v0.23.x branch, and main will shortly become the default branch in the tendermint-rs repo. The current master branch will be kept as-is as a reference branch going forward, such that we can port specific changes to main.

The v0.24.x pre-release crates will be kept around for a while, but will eventually be yanked. Current ETA on this is the end of September 2022, unless anyone objects strongly.

Follow along with #1168 to keep track of important upcoming changes to tendermint-rs. Until the 1.0 release, the next breaking release of tendermint-rs will start at v0.25.0.