REGTEST: make the PROXY TLV validation depend on version 2.2

Regtest proxy_protocol_tlv_validation was added by commit 488ee7fb6e
("BUG/MAJOR: proxy_protocol: Properly validate TLV lengths") but it
relies on a trick involving http-after-response to append a header
after a 400-badreq response, which is not possible in earlier versions,
so make it depend on 2.2.

reg-tests/connection/proxy_protocol_tlv_validation.vtc

@@ -1,5 +1,7 @@
 varnishtest "Check that the TLVs are properly validated"
 
+#REQUIRE_VERSION=2.2
+
 feature ignore_unknown_macro
 
 # We need one HAProxy for each test, because apparently the connection by