Summary:
Similar fix already made for ASAN build. The test still used 50 thread for TSAN
build. It was reported that 50 threads in TSAN build caused other CPU intensive
tests on the same test VM to fail. Reduced to 10 threads for TSAN build as well.

The test runs more flaky under TSAN build with this change, I will look into how
to get rid of the flakiness separately.
Jira: DB-14184

Test Plan: ./yb_build.sh tsan --java-test 'org.yb.pgsql.TestPgDdlConcurrency#testModifiedTableWrite' -n 20 --tp 1

Reviewers: kfranz, mihnea, hsunder

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D40778