update signal error log

[yylt]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Which issue(s) this PR fixes:

Related to kubernetes/kubernetes#120168

Fix #2365

Does this PR introduce a user-facing change?:

when gateway-api-webhook stop by sigterm signal in container, it always exit with -1 code. it will make error containerStatus

the log is here

$ kubectl delete po gateway-api-admission-server-5d555f679-dm27b
pod "gateway-api-admission-server-5d555f679-dm27b" deleted

$ kubectl logs gateway-api-admission-server-6ccfb998f9-r4rfb -f
gateway-api-admission-webhook version: v0.7.1 (ab03a594e7db13b8d2579929b204d8d10990fd2b)
I0828 03:16:37.519872       1 main.go:90] admission webhook server started and listening on :8443
I0828 03:34:08.893252       1 main.go:97] admission webhook received kill signal
F0828 03:34:08.893547       1 main.go:87] admission-webhook-server stopped: http: Server closed

IMO, when process exit, and there are some error happened, it should print with error level, not fatal level

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[k8s-ci-robot]

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[robscott]

Thanks @yylt!

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: robscott, yylt

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[robscott]

/lgtm