WRITE_ENUMS_TO_LOWERCASE must use Locale.ROOT for case folding

[dlwldnjs1009]

Follow-up to #5994 (same root cause as #5993): EnumFeature.WRITE_ENUMS_TO_LOWERCASE
lower-cased enum names with the JVM default Locale, so under Turkish/Azeri locales
IS_ADMIN serialized as ıs_admın instead of is_admin.

#5994 applied Locale.ROOT to the UPPER_SNAKE_CASE / LOWER_CASE naming strategies;
this extends the same fix to the enum lower-casing path.

Changes

• EnumValuesToWrite._fetch: fold enum names with Locale.ROOT (live 3.x value/key path)
• EnumValues._findNameToUse: same fix for the deprecated helper

EnumValues is deprecated on 3.x and no longer used by EnumSerializer directly, but this
keeps the still-public helper locale-stable and matches the 2.x path where EnumValues
remains the live serialization/key path.

Testing

• EnumSerializationTest.testEnumFeature_WRITE_ENUMS_TO_LOWERCASEUsesRootLocale
• EnumValuesTest.testConstructFromNameLowerCasedWithRootLocale

[github-actions]

🧪 Code Coverage Report

Metric	Coverage	Change
Instructions	81.66%	📈 +0.000%
Branches	75.06%	📈 +0.000%

Coverage data generated from JaCoCo test results

[github-actions]

🧪 Code Coverage Report

Metric	Coverage	Change
Instructions	81.66%	📈 +0.000%
Branches	75.07%	📈 +0.010%

Coverage data generated from JaCoCo test results

[github-actions]

🧪 Code Coverage Report

Metric	Coverage	Change
Instructions	81.65%	📉 -0.010%
Branches	75.04%	📉 -0.020%

Coverage data generated from JaCoCo test results

[cowtowncoder]

Whops. Merged concurrent changes that were meant to go in this PR directly in 3.x.
This is why PR may look odd; merging.

[github-actions]

🧪 Code Coverage Report

Metric	Coverage	Change
Instructions	81.66%	📈 +0.010%
Branches	75.07%	📈 +0.030%

Coverage data generated from JaCoCo test results

[cowtowncoder]

Thank you @dlwldnjs1009 !