Rename cilium certgen and envoy image variables

[guoard]

Rename hubble specific cilium image variables to more generic names:

• cilium_hubble_certgen_image_repo → cilium_certgen_image_repo
• cilium_hubble_certgen_image_tag → cilium_certgen_image_tag
• cilium_hubble_envoy_image_repo → cilium_envoy_image_repo
• cilium_hubble_envoy_image_tag → cilium_envoy_image_tag

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
The previous variable names incorrectly implied that these images are
used exclusively by hubble.

certgen is also used by Cluster Mesh

envoy is not only used for hubble. It is also required for L7 network
policy support and is deployed by default on new clusters.

This change makes the variable naming more accurate and aligns it with
their actual usage scope.

Does this PR introduce a user-facing change?:

Rename cilium certgen and envoy image variables

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: guoard
Once this PR has been reviewed and has the lgtm label, please assign yankay for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment