Fix STACKIT documentation issues

[MXClyde]

Summary

This PR is an alternative to #11208 with all review fixes already applied. It targets development directly so the STACKIT documentation can land cleanly without waiting for the original branch to be updated.

It addresses all MUST FIX and SHOULD FIX items from the review on #11208 (see comment #4603492028).

Fixes included

Casing standardization

• Standardize all "StackIt" / "Stackit" mentions to the official STACKIT all-caps spelling across the affected files.
• Rename "Stackit Postgres Flex" to "STACKIT PostgreSQL Flex" in the supported environments list.

Release note (mendix-for-private-cloud.md)

• Replace the one-line STACKIT release note with a complete entry that:
  • Lists the four newly supported STACKIT services (SKE, PostgreSQL Flex, Object Storage, Container Registry) with links.
  • Calls out that customers self-provision SKE / PostgreSQL Flex / Object Storage before deploying Mendix.
  • Documents the two known limitations (CREATEROLE missing on PostgreSQL Flex; missing CreateUser / CreatePolicy / CreateBucket APIs on Object Storage) and how to work around them.

Supported providers (private-cloud-supported-environments.md)

• Add description of the unmanaged provisioning model for SKE.
• Add a statement that any SKE-offered Kubernetes version inside the Supported Versions range is supported.

Storage plans (private-cloud-storage-plans.md)

• Remove the duplicate 3-bullet field list (Endpoint / Access Key / Type) under "Configuring the Plan" for STACKIT Object Storage; the longer field list immediately below already covers those fields.
• Fix grammar: fail to start the and deployment -> fail to start, and the deployment.
• Fix grammar: By creating dedicated bucket for all the environment -> By creating a dedicated bucket per environment.
• Fix double space: Enable this option and the bucket -> Enable this option and the bucket.

Registry (private-cloud-registry.md)

• Revert the unrelated 4-space -> 8-space reindent of the gcloud iam service-accounts add-iam-policy-binding block (lines 200-205) so the diff stays focused on STACKIT changes.
• Fix sentence: the registry created in stackit -> the registry you created in STACKIT.
• Remove trailing whitespace from the STACKIT registry table.

License manager (private-cloud-license-manager.md)

• Revert the unrelated removal of the https<s>://pclm.<domain>/metrics health-check bullet - that endpoint still works and the deletion was outside the scope of the STACKIT PR.

Relationship to #11208

• Same author intent, same files, same content - just with all reviewer feedback already applied.
• Once this PR is merged, #11208 can be closed.

Test plan

• CI lint passes on the updated branch
• Render the affected pages with Hugo and confirm:
  • Release note renders as a bulleted list with working links
  • Supported providers page shows the SKE provisioning note and version statement
  • Storage plans page shows a single field list under "Configuring the Plan" for STACKIT Object Storage
  • Registry page renders the gcloud snippet inside the numbered list (no broken indentation) and the STACKIT registry table has no trailing whitespace
  • License manager page shows both /health and /metrics bullets