GHSA-j88v-2chj-qfwx: pgx: SQL Injection via placeholder confusion with dollar quoted string literals

SQL Injection can occur when:

1. The non-default simple protocol is used.
2. A dollar quoted string literal is used in the SQL query.
3. That string literal contains text that would be would be interpreted as a placeholder outside of a string literal.
4. The value of that placeholder is controllable by the attacker.

e.g.

attackValue := `$tag$; drop table canary; --`
_, err = tx.Exec(ctx, `select $tag$ $1 $tag$, $1`, pgx.QueryExecModeSimpleProtocol, attackValue)

This is unlikely to occur outside of a contrived scenario.