increase timeout for performance tests

[ReToCode]

What this PR does, why we need it:

• Increases the timeout for Serving performance tests
• This is necessary, as I added an additional test case, now the build takes longer than the default 2h

/assign @upodroid

[upodroid]

LGTM

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ReToCode, upodroid

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[knative-prow]

@ReToCode: Updated the job-config configmap in namespace default at cluster default using the following files:

• key serving-main.gen.yaml using file prow/jobs/generated/knative/serving-main.gen.yaml

In response to this:

What this PR does, why we need it:

• Increases the timeout for Serving performance tests
• This is necessary, as I added an additional test case, now the build takes longer than the default 2h

/assign @upodroid

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