Avoid overflow in MaybeRemoveOldWalSummaries().

Avoid overflow in MaybeRemoveOldWalSummaries().

This commit limits the maximum value of wal_summary_keep_time to
INT_MAX / SECS_PER_MINUTE to avoid overflow when it is converted to
seconds.  In passing, use the HOURS_PER_DAY, MINS_PER_HOUR, and
SECS_PER_MINUTE macros in the code for this GUC instead of hard-
coding those values.

Discussion: https://postgr.es/m/20240314210010.GA3056455%40nathanxps13