refactor: deprecate table format

[wangxiaoxuan273]

What this PR does / why we need it:

Deprecate table format, which is only used in oras discover.

When using table with oras discover command:

oras discover -h:

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 #

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 84.40%. Comparing base (dfd8b1e) to head (235a805).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1656      +/-   ##
==========================================
- Coverage   84.42%   84.40%   -0.03%     
==========================================
  Files         125      125              
  Lines        5663     5667       +4     
==========================================
+ Hits         4781     4783       +2     
- Misses        627      629       +2     
  Partials      255      255              

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Pull Request Overview

This PR deprecates the table format used in the oras discover command by issuing a deprecation warning and updating related tests.

• Deprecates table format with a new deprecation message printed via the command's stderr.
• Updates the usage string for table format and adjusts tests to verify the deprecation message.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/oras/internal/option/format.go	Deprecation warning added for table format in flag parsing and usage string updated.
test/e2e/suite/command/discover.go	Test cases modified to reflect the new default and to verify deprecation messaging for table format.

Comments suppressed due to low confidence (2)

cmd/oras/internal/option/format.go:104

• [nitpick] Ensure the deprecation warning message printed here matches the deprecation notice in the usage string for FormatTypeTable for consistency. For example, you might adjust the text so both messages use similar wording regarding the future removal.

if opts.FormatFlag == "table" {

test/e2e/suite/command/discover.go:212

• The test expectation has been updated to check for the deprecation message, which now correctly flags the use of table format. Confirm that 'feature.Deprecated.Mark' is consistently defined to match the actual deprecation message printed.

Expect(err).To(gbytes.Say(feature.Deprecated.Mark))

[Wwwsylvia]

LGTM

[shizhMSFT]

LGTM

[TerryHowe]

/lgtm