Add default value nginx to ingressClassName for the virtualservers CRD

[hanyouqing]

Proposed changes

Fix #6891

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• I have read the CONTRIBUTING doc
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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
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[hanyouqing]

I have hereby read the F5 CLA and agree to its terms

[hanyouqing]

recheck