outdated: add --json flag

[jdx]

Fixes #1766

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.02%	✅ 93.33%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d232859)	16840	14029	83.31%
Head commit (943aa55)	16864 (+24)	14052 (+23)	83.33% (+0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1785)	30	28	93.33%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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_{You may notice some variations in coverage metrics with the latest Coverage engine update. For more details, visit the documentation}