SQL: Unmute TO_CHAR test and remove currently failing cases (#138430)

Fixes #120575
Fixes #126213
Fixes #126494

Unmute `DateTimeToCharProcessorTests`, and remove some of the failing test cases.

Author: ivancea

muted-tests.yml

@@ -60,8 +60,6 @@ tests:
 - class: org.elasticsearch.xpack.ccr.FollowIndexSecurityIT
   method: testCleanShardFollowTaskAfterDeleteFollower
   issue: https://github.com/elastic/elasticsearch/issues/120339
-- class: org.elasticsearch.xpack.sql.expression.function.scalar.datetime.DateTimeToCharProcessorTests
-  issue: https://github.com/elastic/elasticsearch/issues/120575
 - class: org.elasticsearch.xpack.security.authc.service.ServiceAccountIT
   method: testAuthenticateShouldNotFallThroughInCaseOfFailure
   issue: https://github.com/elastic/elasticsearch/issues/120902

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/expression/function/scalar/datetime/DateTimeToCharProcessorTests.java

@@ -33,6 +33,15 @@
 /**
  * Tests the {@link ToCharFormatter} against actual PostgreSQL output.
  *
+ * <p>
+ *     Note: Recreating the data set has some complexities,
+ *     as modern Postgres versions (+12) don't have some of the configured timezones,
+ *     and they also may generate different results for dates before 1900.
+ * </p>
+ * <p>
+ *     Consider if it's worth changing the dataset for this test.
+ * </p>
+ *
  * Process to (re)generate the test data:
  * <ol>
  *     <li>Run the @{link {@link ToCharTestScript#main(String[])}} class</li>