fix: Switch default logging to JSON (issue: 20897)

[teddy-wahle]

Closes #20897

This change updates the default log format in util/log from text to json. It will be used for ArgoCD v3.0.

JSON logging is preferred because it produces structured logs, which are easier to parse and analyze with tools like Datadog and OpenTelemetry (Otel). These integrations rely on structured log formats to extract meaningful data and streamline observability workflows. Switching from json to json ensures better compatibility with modern logging and monitoring solutions.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.42%. Comparing base (bcf2143) to head (6c4f417).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21656      +/-   ##
==========================================
- Coverage   55.45%   55.42%   -0.03%     
==========================================
  Files         339      339              
  Lines       57196    57195       -1     
==========================================
- Hits        31718    31703      -15     
- Misses      22812    22827      +15     
+ Partials     2666     2665       -1     

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[andrii-korotkov-verkada]

Looks like you have to update the sources of the generated code and regenerate it with make.