Use latest MLS

[neekolas]

tl;dr

• Upgrades to the latest MLS which includes some bug fixes
• Moves max_past_epochs values to the configuration.rs file
• Checks is_valid_epoch in line with the MAX_PAST_EPOCHS configuration (now possible with the fixes in OpenMLS)
• Simplifies testing for old epoch messages

[neekolas]

I did some testing with the CLI, where I created a group using main and then switched over to this branch and added more members and sent messages. Everything seems to work as expected.

I'm less confident downgrading would succeed, but that isn't something we need to support.