Update AWS auth docs for SHA-1 deprecation
We now recommend `/rsa2048` as the preferred AWS signature moving
foward, as `/pkcs7` and `/signature` will stop working by default in
Vault 1.12 without setting `GODEBUG=x509sha1=1` in the Vault deployment
due to the move to Go 1.18.
I also took this oppoturnity to try to make the docs less confusing
and more consistent with all of the usages of signature, PKCS#7, DSA,
and RSA terminology.
Co-authored-by: Ben Ash <32777270+benashz@users.noreply.github.com>
Co-authored-by: Theron Voran <tvoran@users.noreply.github.com>
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