chore: Update doc for v2.4.0

[laurentsimon]

How to LGTM this PR (I'll work on a proper doc for this in slsa-framework/slsa-github-generator#112):

1. Clone repo

$ git clone git@github.com:slsa-framework/slsa-verifier.git 
$ cd slsa-verifier
$ bash verify-release.sh v2.4.0 # NOTE: use the file in _this_ PR.
# Note down the path to the temporary dir use. The bash script will print its first line as "INFO: using dir: /tmp/tmp.VaYi6HfbmL"

2. Run command below and compare to SHA256SUM.md in this PR

$sha256sum /tmp/tmp.VaYi6HfbmL/*

The output hash should be the hash I'm updating to in this PR. If they match, LGTM. If they don't, someone tampered with the released binary and don't LGTM

[kpk47]

Verified on OSX.

$ sha256sum /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/*
9e67318937b936014b6127affc14bc45f1fb10d9899b0105877778e8179b3029  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-darwin-amd64
37e7704d26079a78438cb4692817b7ef625077b5f920d753a6f685ffea371b4a  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-darwin-amd64.intoto.jsonl
b55009be65f8f6dae4399522e4ab5685a5cfe0b72dca3134f12ba144b8860607  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-darwin-arm64
fdcbdf306b6c01fa167c50bcaf2503d28fcf9f07c4ff61b4ac1c29611e8d4e33  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-darwin-arm64.intoto.jsonl
9883e4c7fd0fead95815de1533db62d1ae19daf9d333b359e192fc65ffb401b2  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-linux-amd64
cfe3cbd4949b514d55944ace52e627b7550207ef8cebf5bdae22dd0783a2b075  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-linux-amd64.intoto.jsonl
bb025462acd9e93da32694e3ed82bfa57cb487a28c989a083caf2a3569d3cfbe  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-linux-arm64
1e6f7cacdae0399d52492b091a42322fb315bb9a1300f89d9f362940fc19dfd9  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-linux-arm64.intoto.jsonl
a8ea35a4abf450f3828d42cf0b9be3628692508184bec8610a472a7bf4afc843  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-windows-amd64.exe
115b33b8d86ccdde53e3e0c9d35e34f17df71c951e0c67096dd00ad5a1d10ba3  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-windows-amd64.exe.intoto.jsonl
b14cd8228fecabe53e3676ec2d94b53d7aee11f6f5a8dabbe07e840143d48e8d  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-windows-arm64.exe
68df6c068ba97f625d032630b542ef1820d84d7ed14655f2d0b96cc7d295ef3a  /var/folders/70/d2mrl94j0_x1f265cnlhq5d000c_w4/T/tmp.LhxHHDE5/slsa-verifier-windows-arm64.exe.intoto.jsonl