minor: bump indexmap to 2.1.0

[froydnj]

This change aligns the required indexmap version with that of serde_json so there's less crate duplication in the dependency graph.

[froydnj]

(It would be nice if this was landed on the 2.8.x branch too.)

[isabelatkinson]

Hey @froydnj, thanks for making this PR! I ran CI and the minimum supported Rust version (MSRV) compilation task failed due to indexmap 2.1.0 requiring Rust 1.64, which is higher than our current MSRV (1.61). Bumping our MSRV to 1.64 should be fine for the sake of these changes -- would you mind pushing a commit here doing so? The easiest way to make this change is to grep bson-rust for 1.61 and update to 1.64 wherever necessary.

(It would be nice if this was landed on the 2.8.x branch too.)

Per our MSRV policy we don't update the MSRV in patch versions, so we will need to wait to release this until the next minor version of this crate is published.

[froydnj]

@isabelatkinson Thanks for the CI run and the additional context! I pushed a MSRV bump.

[isabelatkinson]

Thanks for the changes @froydnj! I just pushed an update to our CI config and a fix for a clippy failure that came with the MSRV bump. I'm going to tag in another team member for review and then this should be ready to merge.

[abr-egn]

LGTM, thank you!