Clarify that most keys in SARIF are case-sensitive

[jsoref]

GitHub's documentation about SARIF incorrectly used ruleID in a couple of places:

• ruleId is written as ruleID in a couple of places github/docs#35709

In wanting to file that ticket, I tried to find a reference in the official specification to clearly indicate that they were wrong, but I couldn't find one.

https://docs.oasis-open.org/sarif/sarif/v2.1.0/sarif-v2.1.0.html doesn't seem to mention that keys are generally case-sensitive.

There is a note in
https://docs.oasis-open.org/sarif/sarif/v2.1.0/errata01/os/sarif-v2.1.0-errata01-os-complete.html#_Toc141790874 that indicates that something related to hashes may be (it's actually fairly hard to tell that it's just talking about the properties of the hash bag, but that's a separate thing).