Update fips-containers.istio.tetratelabs.com/pilot Docker tag to v1.18.6

This MR contains the following updates:

Package	Type	Update	Change
fips-containers.istio.tetratelabs.com/pilot		patch	1.18.5 -> 1.18.6
fips-containers.istio.tetratelabs.com/pilot	ironbank-docker	patch	1.18.5-tetratefips-v0-distroless -> 1.18.6-tetratefips-v0-distroless
fips-containers.istio.tetratelabs.com/pilot	stage	patch	1.18.5-tetratefips-v0-distroless -> 1.18.6-tetratefips-v0-distroless

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