net-im/dendrite: Pin Go version to 1.19

vendor/github.com/lucas-clemente/quic-go/internal/qtls/go120.go:5:13: cannot use "The version of quic-go you're using can't be built on Go 1.20 yet.

quic-go doesn't support go1.20 yet, keep building this port with go1.19
for now.

PR: 269289
Reported by: exp-run