[release-1.0], kind: Pin metallb to v0.14.8

[qinqon]

What this PR does and why is it needed

There are some expectation at the dev-env interface at metallb that can change and break ovn-k CI, let's pin it so we can propertly consume those changes at a PR later on.

(cherry picked from commit b3ba566)

[release-1.0], kind: Pin metallb to v0.14.8

[qinqon]

failing at
``
2024-09-11T08:27:06.4723114Z �[38;5;9m[FAILED] failed to create secondary network "secondary-network" with subnet(s) 2001:db8:abcd:1234:c000::/64: failed to run "docker network create --internal --driver bridge secondary-network --subnet 2001:db8:abcd:1234:c000::/64 --ipv6": exit status 1 (Error response from daemon: invalid network config:
2024-09-11T08:27:06.4725952Z invalid subnet 2001:db8:abcd:1234:c000::/64: it should be 2001:db8:abcd:1234::/64

[qinqon]

failing flakes:

• control-plane: [release-1.0] should work on secondary node interfaces for ETP=local and ETP=cluster when backend pods are also served by EgressIP #4719
• external-gateway: Flake: [FAIL] External Gateway With Admin Policy Based External Route CRs e2e non-vxlan external gateway through a gateway pod Should validate ICMP connectivity to an external gateway's loopback address via a gateway pod [It] ipv4 #4474
• shared-conformance: CI flake: shard-conformance timeout v2 #4340