Update MetalLB version to 0.15.3 to provide recent FRR version for frr-k8s
FRR versions bundled with MetalLB releases 0.14.9 and 0.15.2 are affected by CVEs - https://github.com/metallb/metallb/issues/2893.
MetalLB 0.15.3 addresses these issues by upgrading the FRR version used in frr-k8s mode to 10.4.1 (August 2025).