CLOUDP-364847: Remove unused MDB_IMAGE_TYPE environment variable

[Julien-Ben]

Summary

The MDB_IMAGE_TYPE environment variable was originally used by the community operator but became orphaned after the MEKO/MCO merge. The community operator now uses MDB_COMMUNITY_IMAGE_TYPE instead. It was used by the enterprise operator until around October 2024.
Commit removing its usage: https://github.com/10gen/ops-manager-kubernetes/commit/e0127188c835c2bb374488533950824180faf490
And then, used by the community operator only.

This PR removes the unused variable from:

• Helm chart values and templates
• Operator deployment configs
• Dev and E2E test scripts
• Go code (unused constant)

As a follow up of this ticket, I created WRITING-34891, to come up with an engineering proposal, which will propose a proper solution for configurable UBI base image selection.

Note: I added the documentation changes required in the Jira ticket, for public doc.
For example we mention it here: https://www.mongodb.com/docs/kubernetes/current/reference/kubectl-operator-settings/

Proof of Work

• Builds successfully and unit tests pass.
• Public manifests regenerated via make precommit

Checklist

• Have you linked a jira ticket and/or is the ticket in the title?
• Have you checked whether your jira ticket required DOCSP changes?
• Have you added changelog file?
  • use skip-changelog label if not needed
  • refer to Changelog files and Release Notes section in CONTRIBUTING.md for more details

[github-actions]

⚠️ (this preview might not be accurate if the PR is not rebased on current master branch)

MCK 1.6.1 Release Notes

Bug Fixes

• Backed up the agent password in a secret for SCRAM authentication to prevent unnecessary password rotations.
• MongoDB Adding missing ownerrefs to ensure proper resource deletion by kubernetes.
• Single Cluster Deleting resources created by CRD now only happens on multi-cluster deployments. Single Cluster will solely rely on ownerrefs.
• MongoDB, MongoDBOpsManager: Improve validation for featureCompatibilityVersion field in MongoDB and MongoDBOpsManager spec.
  The field now enforces proper semantic versioning. Previously, invalid semver values could be accepted,
  potentially resulting in incorrect configurations.

Other Changes

• Operator configuration: Removed the unused MDB_IMAGE_TYPE environment variable and the corresponding mongodb.imageType Helm value. This variable was deprecated in v1.28.0 of the MongoDB Enterprise Kubernetes Operator when it switched to architecture-based image selection (ubi9 for static, ubi8 for non-static). This is a cleanup change with no functional impact.

[Julien-Ben]

This is a reference architecture file

[Julien-Ben]

@vinilage I will merge once I have your approval for the release notes