move docker registry login values to repository environment

[WindzCUHK]

Description

as title

Changes

• remove unused DELETE_UNTAGGED_IMAGES_TOKEN and DELETE_UNTAGGED_IMAGES_PER_PAGE
• move DOCKER_REGISTRY_USER and DOCKER_REGISTRY_TOKEN_NAME to repository environment
• rename DOCKER_REGISTRY_ORG => DOCKER_REGISTRY_ORG

Type of change

• Bug fix
• New feature
• Refactoring (no functional changes, no api changes)
• Non-code changes (update documentation, pipeline, etc.)

Flags

• Breaks backward compatibility
• Requires a documentation update
• Has untestable code

---

Checklist

• Followed the guidelines in the CONTRIBUTING document
• Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• Tested and linted the code
• Commented the code
• Made corresponding changes to the documentation
• Passed all pipeline checking

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip]
• Delete the branch after merge

[mlajkim]

OK

Identitcal change with
https://github.com/AthenZ/athenz-client-sidecar/pull/17/files
https://github.com/AthenZ/authorization-proxy/pull/29/files
https://github.com/AthenZ/garm/pull/32/files