Fix case-insensitive Strings.replace dropping length-changing matches

[alhudz]

Repro: Strings.CI.replace("aİb", "İ", "X", -1) returns aİb instead of aXb under any non-Turkish locale (StringUtils.replaceIgnoreCase/replaceOnceIgnoreCase delegate here, so they are affected too).
Cause: replace lower-cases the search argument up front, but the case-insensitive indexOf it calls already matches case-insensitively through CharSequenceUtils.regionMatches. 'İ' (U+0130) lower-cases to the two-char "i̇", so both the needle and the derived replLength grow to length 2 and stop lining up with the single source character, so the match is silently dropped.
Fix: remove the redundant lower-casing and let the case-insensitive indexOf do the matching. The case-sensitive path never lower-cased, so it is unchanged.

The regression test pins the locale with @DefaultLocale("en") so the lower-case expansion is deterministic; it fails on the current code and passes with the fix.

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[garydgregory]

Thank you @alhudz , merged 🚀