corendns: Skip not A Target related labels is forgot during adding owner

[farodin91]

What happened:

• during adding for the owner strict flag
• a new label was introduce and added to endpoints
• certain labels was filter prevent unexpected delete calls, see 83ea480

What you expected to happen:

• no delete call for owner labels on an Endpoint

How to reproduce it (as minimally and precisely as possible):

{"level":"info","msg":"Delete key /skydns/local/domain1/test-owner","time":"2026-02-18T12:13:49Z"} 

Anything else we need to know?:

#6206

Environment:

• External-DNS version (use external-dns --version): 0.21.0
• DNS provider: coredns
• Others: --txt-owner-id=test-owner --coredns-strictly-owned

Checklist

• I have searched existing issues and tried to find a fix myself
• I am using the latest release,
  or have checked the staging image to confirm the bug is still reproducible
• I have provided the actual process flags (not Helm values)
• I have provided full external-dns debug logs
• I have described what DNS records exist and what I expected