ICU-23108 Add Gregorian eras to Japanese calendar

[robertbastian]

Work in progress

Checklist

• Required: Issue filed: ICU-23108
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/test/intltest/incaltst.cpp is different
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/calendar/IBMCalendarTest.java is no longer changed in the branch
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/calendar/JapaneseTest.java is no longer changed in the branch

View Diff Across Force-Push

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