Add apiVersion: v1 to gardener-extension-admission-cilium ServiceAccount

[plkokanov]

How to categorize this PR?

/area networking
/kind bug

What this PR does / why we need it:
With #234 the apiVersion field was removed from charts/gardener-extension-admission-cilium/charts/runtime/templates/serviceaccount.yaml.

This causes the following error when using these charts to deploy the admission with the local gardener setup with extensions:

[error validating data: apiVersion not set; if you choose to ignore these errors, turn validation off with --validate=false](error: error validating "/var/folders/bh/hg7dql3d335b8bbcfz7t242h0000gn/T/tmp.b0KmJ2olbv/runtime-resources.yaml": error validating data: apiVersion not set; if you choose to ignore these errors, turn validation off with --validate=false)

Which issue(s) this PR fixes:
None

Special notes for your reviewer:

Release note:

Fixed a validation error which occurs when deploying the `gardener-extension-admission-cilium` charts because of a missing `apiVersion` field in its `charts/gardener-extension-admission-cilium/charts/runtime/templates/serviceaccount.yaml` file.

[DockToFuture]

/lgtm

[gardener-prow]

@plkokanov: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-extension-networking-cilium-e2e-kind	fb85f8e	link	true	/test pull-extension-networking-cilium-e2e-kind

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

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