Use golang v1.18 for apidiff test

[acumino]

What this PR does / why we need it:
This PR updated apidiff test job to use golang v1.18. This is required cause there is no release available of go-apidiff which supports golang v1.19.
There is an open PR on go-apidiff side to support the new version of go joelanford/go-apidiff#19.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

[acumino]

/kind bug

[oliver-goetz]

/approve
/lgtm

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: oliver-goetz

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[gardener-prow]

@acumino: Updated the job-config configmap in namespace prow at cluster gardener-prow-trusted using the following files:

• key gardener-apidiff.yaml using file config/jobs/gardener/gardener-apidiff.yaml

In response to this:

What this PR does / why we need it:
This PR updated apidiff test job to use golang v1.18. This is required cause there is no release available of go-apidiff which supports golang v1.19.
There is an open PR on go-apidiff side to support the new version of go joelanford/go-apidiff#19.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

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