Use jq --compact-output option in the binding update command

[shafeeqes]

How to categorize this PR?

/area control-plane-migration
/kind bug

What this PR does / why we need it:
After #6727, the command

gardener/docs/usage/control_plane_migration.md

Line 34 in 779f0bf

kubectl -n ${NAMESPACE} get shoot ${SHOOT_NAME} -o json | jq -c '.spec.seedName = "<destination-seed>"' | kubectl replace --raw /apis/core.gardener.cloud/v1beta1/namespaces/${NAMESPACE}/shoots/${SHOOT_NAME}/binding -f - | jq -r '.spec.seedName'

throws error:

Error from server (Forbidden): shoots.core.gardener.cloud "shoot" is forbidden: only spec.seedName can be changed using the binding subresource when the shoot is being rescheduled to a new seed

After checking what's the spec change, it's only how it is represented by jq.
This PR adds -c or --compact-output to the jq command, so that the expected and provided format remains same.

I have also added a validation to reject the update of binding if seedName is same.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
/cc @plkokanov

Release note:

The documentation for triggering control-plane migration is updated with a slight change.

[plkokanov]

After digging some more, jq was not the only thing that was causing the problem. Here's a more detailed description of what was happening and why the updated was rejected by the admission plugin:
There are fields in the shoot which are of type RawExtension. These fields will be represented as []byte arrays containing (most likely) some json data and the keys in that data might not be sorted. Additionally that data does not contain any new line characters.
However, when you retrieve the shoot via kubectl get shoot -o json the json output will sort all keys and properly format the json output with new lines so that the output is easier to read. This also includes json objects that are in the RawExtension fields.
When the shoot is reapplied with kubectl replace --raw, the sorted json object with the new lines will be passed to the apiserver which means that the []byte RawExtensions will get populated with the passed-in data directly - containing sorted keys and new lines. This will differ from the []byte array that of the old object has during the admission checks and therefore the update will be forbidden.

Using jq -c is not enough due to the sorted keys retrieved when calling kubectl get ... -o json. Instead the following command should do the trick:

kubectl get --raw /apis/core.gardener.cloud/v1beta1/namespaces/${NAMESPACE}/shoots/${SHOOT_NAME} | jq -c '.spec.seedName = "<destination-seed>"' | kubectl replace --raw /apis/core.gardener.cloud/v1beta1/namespaces/${NAMESPACE}/shoots/${SHOOT_NAME}/binding -f - | jq -r '.spec.seedName'

[gardener-prow]

@shafeeqes: 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-gardener-check-vulnerabilities	238f6fa	link	false	/test pull-gardener-check-vulnerabilities

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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[plkokanov]

Thanks!

/lgtm
/approve

[gardener-prow]

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