WIP: migrate deprecated datacenter field to location

[buraksekili]

What this PR does / why we need it:

Hetzner removed the datacenter property from its Servers API in July 2026, so any MachineDeployment that still sets cloudProviderSpec.datacenter either fails to provision or lands in Hetzner's default location. This PR migrates datacenter to location on admission by deriving the location from the datacenter name prefix (nbg1-dc3 becomes nbg1), removes all calls to the removed datacenter endpoints, and keeps the field parseable through a deprecation window so existing specs do not break on strict unmarshal. Hard removal of the field is left for a later release.

Which issue(s) this PR fixes:
Fixes #2053

What type of PR is this?
/kind bug
/kind deprecation

Special notes for your reviewer:

Numeric datacenter values are rejected instead of migrated because Hetzner datacenter IDs do not map to location IDs (datacenter 4 is fsn1-dc14 while location 4 is ash). Create derives the location itself for specs that bypass the webhook, so runtime placement stays correct even with the webhook disabled. hcloud-go is bumped from v2.13.1 to v2.44.0 with no other dependency churn and no call site changes. The new TestHetznerProvisioningE2EDatacenterMigration runs in the existing hetzner presubmit because its name matches the job's -run filter. The dc metrics label is gone; dashboards using it need updating.

Does this PR introduce a user-facing change? Then add your Release Note here:

machine-controller no longer sends the removed `datacenter` field to the Hetzner API. Hetzner specs setting `cloudProviderSpec.datacenter` are migrated to `location` automatically on admission, and the `dc` metrics label was removed. Update your specs to use `location`; the `datacenter` field will be removed in a future release.

Documentation:

NONE

Test issue:

Prerequisites: cluster running this machine-controller build with a Hetzner project token.

1. Create a MachineDeployment with cloudProviderSpec.datacenter set to nbg1-dc3 and no location.
2. Verify the webhook rewrote the spec: location is nbg1 and datacenter is empty.
   get machinedeployment <name> -o jsonpath='{.spec.template.spec.providerSpec.value.cloudProviderSpec}'
3. Verify the server is created in nbg1 and the node joins the cluster.
4. Create a MachineDeployment with only location set to fsn1 and verify it provisions unchanged.

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