🌱 test/e2e: bump WorkloadKubernetesVersion for v1.6 clusterctl upgrade test

[chrischdi]

What this PR does / why we need it:

Part of #9578

Bumps the k8s version used because the test now uses CAPI v1.6.1 which supports v1.29.0.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

/area ci

[chrischdi]

/test help

[k8s-ci-robot]

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[chrischdi]

/test pull-cluster-api-e2e-full-main

[sbueringer]

Thank you!

/lgtm
/approve

[k8s-ci-robot]

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This pull-request has been approved by: sbueringer

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[k8s-ci-robot]

LGTM label has been added.

Git tree hash: faf42d14c09de25ae41e2cb39af1fa9201938961