Update docker image for bridges substrate-relay (#5947)

This PR updates the substrate-relay version for the bridges' Zombienet
tests.

docker/dockerfiles/bridges_zombienet_tests_injected.Dockerfile

 # this image is built on top of existing Zombienet image
 ARG ZOMBIENET_IMAGE
 # this image uses substrate-relay image built elsewhere
-ARG SUBSTRATE_RELAY_IMAGE=docker.io/paritytech/substrate-relay:v1.6.10
+ARG SUBSTRATE_RELAY_IMAGE=docker.io/paritytech/substrate-relay:v1.7.0
 
 # metadata
 ARG VCS_REF