Summary:
The pg15 allows parallel query in serializable isolation, however it
fails an assertion later on. For a quick fix disable the parallel
query in serializable isolation again. GHI #23549 is created for proper
fix.

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestPgTransparentRestarts#parallel'

Reviewers: jason, tfoucher

Reviewed By: tfoucher

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D37386