Adjust the regex for load balancer spec

What does this MR do?

Fixes #324682 (closed)

The pg_lsn type is a hexadecimal which would include characters 0-9 and A-F. The regex only handed 0-9 and A-E and the failures were due to F being in (which looks valid to me.

See #324682 (comment 529708852) for more details

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Related to #324682 (closed)