Revert "Upgrade parent pom version from 31 to 33"

This reverts commit fc262c03810a56b5783883a76056a76daac2e3e8.

Important Read

Please ensure the GitHub issue is mentioned at the beginning of the PR

What is the purpose of the pull request

https://github.com/apache/incubator-xtable/issues/486 mvn deploy -Prelease -DskipTests -DdeployArtifacts=true is failing for version 33 as it's not supporting java 1.8 docs. Looked up other OSS projects and official documentation we can use any version greater than 22. https://maven.apache.org/pom/asf/

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.8.0:jar (attach-javadocs) on project xtable-api: MavenReportException: Error while generating Javadoc: 
[ERROR] Exit code: 1
[ERROR] /Users/vinishreddy/helm/configs/incubator-xtable/xtable-api/target/delombok/org/apache/xtable/spi/extractor/ExtractFromSource.java:38: error: method references are not supported in -source 7
[ERROR]     Iterator<TableChange> tableChangeIterator = commitsBacklog.getCommitsToProcess().stream().map(conversionSource::getTableChangeForCommit).iterator();
[ERROR]                                                                                                                     ^
[ERROR]   (use -source 8 or higher to enable method references)
[ERROR] /Users/vinishreddy/helm/configs/incubator-xtable/xtable-api/target/delombok/org/apache/xtable/spi/sync/TableFormatSync.java:64: error: lambda expressions are not supported in -source 7
[ERROR]         results.put(conversionTarget.getTableFormat(), getSyncResult(conversionTarget, SyncMode.FULL, internalTable, target -> target.syncFilesForSnapshot(snapshot.getPartitionedDataFiles()), startTime, snapshot.getPendingCommits()));

Brief change log

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Revert back apache parent pom version to 31 as mvn deploy is failing

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This pull request is a trivial rework / code cleanup without any test coverage.

apache / incubator-xtable