fix: force config media type when setting platform

[qweeah]

What this PR does / why we need it:

When selecting a platform, oras-go only allows config type to be application/vnd.oci.image.config.v1+json. See https://github.com/oras-project/oras-go/blob/v2.5.0/internal/platform/platform.go#L110.

This PR enforces above design with 2 changes:

1. config.mediaType is set to application/vnd.oci.image.config.v1+json when baking platform into the config blob;
2. The usage of oras push --artifact-type xxx -artifact-platform xxx --image-spec v1.0 ... is banned.

Without this PR, oras push might generate artifacts that cannot be consumed by oras pull --platform, see #1517 (comment)

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 #1517

Please check the following list:

• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.87%. Comparing base (9a83394) to head (7ac6dad).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1519   +/-   ##
=======================================
  Coverage   83.86%   83.87%           
=======================================
  Files         118      118           
  Lines        5164     5166    +2     
=======================================
+ Hits         4331     4333    +2     
  Misses        592      592           
  Partials      241      241           

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[TerryHowe]

/lgtm