(maint) Remove misleading comment regarding msExtReq

Puppet has never provided a way to add microsoft extension requests to
generated CSRs, so it's not something we would necessarily restore. This
commit removes a comment that indicates that we used to support
msExtReq.
(#23201) Allow CSRs to return multiple extension requests
(#23201) Extract CSR extension_request attr generation
(#23201) allow custom fields to be added to CSR extReq
(#23201) don't assign OIDs when looking up extension requests

The `Puppet::SSL::CertificateRquest#subject_alt_names` method was trying to
select all of the subjectAltName extensions in the extension requests,
but was accidentally assigning that OID to all extensions instead of
comparing them.

Before 63cecc56 subjectAltName was the only possible extension request so
this was innocuous, but following that commit this error was destructive.
This commit resolves the issue by using the comparison operator instead
of assignment.