Ratatui is a maintained fork of tui-rs. See https://ratatui.rs/ for more
information.

Changes the backend to use crossterm instead of termion. This makes it
possible to use the terminal UI on Windows.

The rust-toolchain.toml config sets the version used to compile as 1.74,
while the MSRV for the rest of the workspace is 1.70. This is to allow
metrics-observer to build, as it requires 1.74.

This commit adds a matrix setting to the CI workflow to allow testing
the MSRV separately from the rest of the workspace, and some additional
documentation to explain the discrepancy.