Add a presubmit CI check for LTS

For dependency upgrades in the LTS bom(e.g. https://github.com/GoogleCloudPlatform/cloud-opensource-java/pull/2466), there should be a CI check to make sure there are no dependencies being accidentally downgraded. This has been done manually by running `mvn install -Dtest='BomContentTest#testLtsBom'` locally.