feat: make all go-template key lower-cased

[qweeah]

What this PR does / why we need it: This PR makes all formattable names of the metadata output lower cased.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #1342

Please check the following list:

• [x] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [ ] Does this change require a documentation update?
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
• [ ] Do all new files have an appropriate license header?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

:exclamation: No coverage uploaded for pull request base (main@cc74e50). Click here to learn what that means.

:exclamation: Current head 72a04da differs from pull request most recent head d743e72. Consider uploading reports for the commit d743e72 to get more accurate results

Files	Patch %	Lines
cmd/oras/internal/display/utils/json.go	75.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1343 +/- ## ======================================= Coverage ? 84.50% ======================================= Files ? 103 Lines ? 3653 Branches ? 0 ======================================= Hits ? 3087 Misses ? 337 Partials ? 229 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.