fix: use future dates in checkpoint prune tests to prevent time-dependent failures (#5543)

The test_older_than tests in both JSON and SQLite prune suites used
hardcoded 2026-04-17 timestamps for the 'new' checkpoint. Once that
date passes, the checkpoint is older than 1 day and gets pruned along
with the 'old' one, causing assert count >= 1 to fail (count=0).

Use 2099-01-01 for the 'new' checkpoint so tests remain stable.

Co-authored-by: Joao Moura <joaomdmoura@gmail.com>