refactor(dp): remove readiness TCP fallback

[Automaat]

Motivation

Unix socket is the default readiness reporter mode since v2.9.0. Remove the TCP fallback (ReadinessUnixSocketDisabled, ReadinessPort) and the feature flag (FeatureReadinessUnixSocket) that guarded it. Unix socket is now the only mode.

Implementation information

• Remove ReadinessUnixSocketDisabled and ReadinessPort from Dataplane config and bootstrap request
• Remove FeatureReadinessUnixSocket feature flag constant
• Remove FieldDataplaneReadinessPort metadata field and GetReadinessPort()
• admin_proxy_generator now unconditionally uses unix socket endpoint for readiness cluster
• Readiness component always starts unix socket listener; TCP path deleted
• K8s injector no longer excludes TCP readiness port from iptables; PostStart hook uses --unix-socket when envoyAdminUnixSocket=true
• Ingress/egress helm charts: replaced KUMA_READINESS_UNIX_SOCKET_DISABLED + KUMA_READINESS_PORT with KUMA_DATAPLANE_RUNTIME_SOCKET_DIR

Changelog: refactor(dp): remove readiness TCP fallback

Closes #14039

[github-actions]

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• Is the PR title satisfactory? Is this part of a larger feature and should be grouped using > Changelog?
• PR description is clear and complete. It Links to relevant issue as well as docs and UI issues
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as an image registry)
• IPv6 is taken into account (.e.g: no string concatenation of host port)
• Tests (Unit test, E2E tests, manual test on universal and k8s)
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