CVE-2023-29409

Extremely large RSA keys in certificate chains can cause a client/server to
expend significant CPU time verifying signatures. With fix, the size of RSA
keys transmitted during handshakes is restricted to <= 8192 bits. Based on
a survey of publicly trusted RSA keys, there are currently only three
certificates in circulation with keys larger than this, and all three
appear to be test certificates that are not actively deployed. It is
possible there are larger keys in use in private PKIs, but we target the
web PKI, so causing breakage here in the interests of increasing the
default safety of users of crypto/tls seems reasonable.