Handle singular case for incompatible rules warning

[RubenVanEldik]

When running Ruff format I got a slightly confusing message. The Ruff message was confusing, because it was in plural, even though only a single rule was giving me this error.

Summary

• Updated the code to call warn_user_once! twice, handling both singular and plural cases separately.
• Ensures the warning message is grammatically correct when only one incompatible rule is present.

Test Plan

It's a minor change, so the automatic tests should cover this.

[MichaReiser]

Thanks

[codspeed-hq]

CodSpeed Performance Report

Merging #13212 will improve performances by 4.9%

_{Comparing RubenVanEldik:dont-use-plural-when-single-rule-warning (a1e5734) with main (0f85769)}

Summary

⚡ 1 improvements
✅ 31 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	RubenVanEldik:dont-use-plural-when-single-rule-warning	Change
⚡	linter/all-with-preview-rules[numpy/globals.py]	866.2 µs	825.7 µs	+4.9%

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.