Bump micromamba to support new nextstrain/conda-base package

[joverlee521]

Context

Full context in nextstrain/conda-base#105

Summary: If nextstrain/conda-base includes package that specifies x86_64-microarch-level, then installation will fail due to bug in mamba, which has been resolved in v1.5.10 and v2.0.0..

So we have the option to bump to 1.5.10 or 2.0.0.

[joverlee521]

From mamba's README:

Only mamba and micromamba 2.0 and later are supported and are actively developed.

The 1.x branch is only maintained for addressing security issues such as CVEs.

So if we have to bump the version, might as well bump to v2.

[joverlee521]

Major changes listed in https://mamba.readthedocs.io/en/latest/developer_zone/changes-2.0.html do not seem to affect us.

[joverlee521]

Skimmed over CHANGELOG from 1.5.1 to 2.1.0 and nothing jumped out to me.

Will make PR to bump micromamba version to 2.1.0