Remove YAML merge and variable from config YAML

[imjasonh]

Fixes #3794

This enables kustomize and ytt users to more easily consume and modify the YAML.

/kind feature

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• [n] Includes tests (if functionality changed/added)
• [n] Includes docs (if user facing)
• [y] Commit messages follow commit message best practices
• [y] Release notes block has been filled in or deleted (only if no user facing changes)

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
  the release Task to build and release this image.

Reviewer Notes

If API changes are included, additive changes must be approved by at least two OWNERS and backwards incompatible changes must be approved by more than 50% of the OWNERS, and they must first be added in a backwards compatible way.

Release Notes

Make release.yaml more easily consumed by tools like kustomize and ytt

[imjasonh]

Note: the merge syntax was added in #3236

[pritidesai]

/lgtm

Makes it too repetitive and that's the reason #3236 had implemented configs with merge key. But, to make them consumable by other tools, we need these changes.

[vdemeester]

So, it's a shame not all tools support all features of yaml 😅

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [vdemeester]

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

[bobcatfish]

heeeeey @imjasonh i was randomly looking at commit messages in the repo (may have been to settle a bet which i then immediately lost) but i noticed that this has a commit message that just says "Remove YAML merge and variable from config YAML " and i want to give a plug for including more context in commit messages (even the PR description has a bit more context "This enables kustomize and ytt users to more easily consume and modify the YAML." tho as a random reader i still dont really understand why theses changes make it so that this is easier for these tools)

apologies for the nag 🙏