Reference beta by version rather than date

[Skgland]

Since¹ this guide has been written

rust-forge/src/release/crater.md

Lines 16 to 18 in c780573

	$STABLE is e.g. 1.39.0 (the stable release)
	$BETA is beta-YYYY-MM-DD, get the date by looking at https://static.rust-lang.org/manifests.txt and
	get the date of the most recent channel-rust-beta.toml.

It is now also possible to specify beta version by their version instead of just by date i.e. 1.75.0-beta for latest beta of 1.75.0 or 1.75.0-beta.1 for the first beta release of 1.75.0.

So would it make sense to simplify this guide to

$STABLE is e.g. `1.80.0` (the stable release) and $BETA is `1.81.0-beta.1`. 

Footnotes

1. the release process started creating the necessary channel files with beta-1.75.0-beta.1 and rustup gained to capability to use those in rustup-1.28.0 ↩