Dependency updates

[wjwoodson]

This PR includes two three updates to resolve issues recently encountered when setting up a new debian:buster-image based installation:

1. Latest Flask library is no longer compatible with Werkzeug<1.0.0 (at least since Flask 2.0.0). The changes here remove the Werkzeug version pin from requirements, and make a small update to use cachelib for functionality extracted to its own library (ref).
2. Small change to migrate jinja2.Markup() usage to markupsafe.Markup() for similar library deprecations (ref).
3. Updates the MySQL connection string in README to specify using the pymysql driver included in the requirements file (ref).

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[gwarf]

It's also required to update send in scoreboard/attachments/file.py as due to download_name replacing Flask's attachment_filename: https://github.com/pallets/werkzeug/blob/d36aaf12b5d12634844e4c7f5dab4a8282688e12/src/werkzeug/utils.py#L400