go121: Update to 1.21.9.

This minor release includes 1 security fix following the security policy:

http2: close connections when receiving too many headers

Maintaining HPACK state requires that we parse and process all HEADERS and
CONTINUATION frames on a connection. When a request's headers exceed
MaxHeaderBytes, we don't allocate memory to store the excess headers but we do
parse them. This permits an attacker to cause an HTTP/2 endpoint to read
arbitrary amounts of header data, all associated with a request which is going
to be rejected. These headers can include Huffman-encoded data which is
significantly more expensive for the receiver to decode than for an attacker to
send.

Set a limit on the amount of excess header frames we will process before
closing a connection.

Thanks to Bartek Nowotarski (https://nowotarski.info/) for reporting this
issue.

This is CVE-2023-45288 and Go issue https://go.dev/issue/65051.

(bsiegert)