Upgrade jackc/pgproto3

[dan-j]

jackc/pgproto3#35 introduces a breaking change but fixes a security vulnerability.

Dependabot has created these PRs but builds are failing:

• Bump github.com/jackc/pgx/v4 from 4.13.0 to 4.18.2 #31
• Bump github.com/jackc/pgproto3/v2 from 2.1.1 to 2.3.3 #32