Add option to remove legacy Tiller server

What does this MR do?

The idea is to simplify the removal of a legacy Tiller server from a cluster by offering a small UI for it. Otherwise, the user needs access the cluster externally with a compatible version of Helm, and either

• start a local Tiller server,
• or use the correct TLS certs for auth

to run helm reset --force, which deletes the Tiller server while leaving the release data intact.

Screenshots (strongly suggested)

Manual QA

Preparation

To simulate a legacy cluster, I installed Tiller using the deprecated installation code.

c = Clusters::Cluster.last # cluster I just attached

# simulate old cluster
c.update(helm_major_version: 2)

# get root user
u = User.first

# install helm
Clusters::Applications::CreateService.new(c.reload, u, application: 'helm').execute({})

Test steps

1. Install Runner ✅
2. Uninstall Helm ✅
3. Run helm2 ls against gitlab-managed-apps, runner should still be there ✅
4. Uninstall Runner ✅
5. Install Runner ✅

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team