Fix locale for monthName/dayName function

[bugmakerrrrrr]

Description

The monthName/dayName function uses Locale.getDefault to get the display name, which may return different results in different env and cause a test error.

org.opensearch.sql.sql.DateTimeFunctionIT > testMonthName FAILED
    java.lang.AssertionError: 
    Expected: iterable with items [[September]] in any order
         but: not matched: <["九月"]>
        at __randomizedtesting.SeedInfo.seed([3027A175091234F:5336A188EBBE09E2]:0)
        at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:18)
        at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:6)
        at org.opensearch.sql.util.MatcherUtils.verify(MatcherUtils.java:173)
        at org.opensearch.sql.util.MatcherUtils.verifyDataRows(MatcherUtils.java:149)
        at org.opensearch.sql.sql.DateTimeFunctionIT.testMonthName(DateTimeFunctionIT.java:801)

Issues Resolved

[List any issues this PR will resolve]

Check List

• [x] New functionality includes testing.
  • [x] All tests pass, including unit test, integration test and doctest
• [x] New functionality has been documented.
  • [x] New functionality has javadoc added
  • [x] New functionality has user manual doc added
• [x] Commits are signed per the DCO using --signoff

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