RFR: 1519: MailmanListReader misses new month on the 1st

Mon Aug 1 21:38:33 UTC 2022

After expanding the tests for MailmanListReader in [SKARA-1513](https://bugs.openjdk.org/browse/SKARA-1513), I've discovered that some tests started failing on the 1st of the month. These failures actually uncovered a real problem, where the current logic would not start querying a new month archive until the 2nd of the new month. This is basically an off by one error in the loop condition where we generate the list of months to iterate over.

-------------

Commit messages:
 - SKARA-1519

Changes: https://git.openjdk.org/skara/pull/1345/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1345&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1519
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1345.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1345/head:pull/1345

PR: https://git.openjdk.org/skara/pull/1345