[stable/gemini] Add priorityClassName for Gemini

[DeD1rk]

Why This PR?
This PR adds support for setting a priority class for Gemini pods.

Fixes #1724

Changes
Changes proposed in this pull request:

• Added `priorityClassName` parameter to values.yaml with appropriate documentation
• Updated deployment template to conditionally set priorityClassName in pod spec
• Bumped chart version from 2.1.3 to 2.2.0 (minor version for new feature)
• Updated README.md with the new parameter documentation

Checklist:

• I have included the name of the chart in the title of this PR in square brackets i.e. [stable/goldilocks].
• I have updated the chart version in Chart.yaml following Semantic Versioning.
• Any new values are backwards compatible and/or have sensible default.
• Any new values have been added to the README for the Chart, or helm-docs --sort-values-order=file has been run for the charts that support it.

[CLAassistant]

All committers have signed the CLA.

[DeD1rk]

@sudermanjr could you take a look? I'll use a post renderer to set this for now, but I think people can benefit from seeing that this setting exists through the values file.

[sudermanjr]

LGTM. Thanks!