We've one index gatling-data-2021-11 that has too much data.
Currently it has 134.5gb.

We want it split into monthly chunks as we think this one large index will
make upgrades difficult.

• reindex 2021-11 data that fits only in November into a temp 2021-11
• delete the orig 2021-11
• reindex the temp 2021-11 back to the orig name
• clean up gatling-data which means reindexing any data that is in it, then deleting it and immediately completing Gatling data retention #249