chore: drop .tool-versions (#88)

The gemspec requires Ruby >= 3.2 and CI tests 3.2 through 4.0, so a single
pinned patch release here says nothing useful. It read `ruby 2.7.7` -- two whole
series below the floor, so anyone setting this repository up with asdf or mise
got a Ruby that cannot install the gem under development.

busser itself carries no .tool-versions, so this also brings the repository in
line with the rest of the family.

Signed-off-by: Tim Smith <tsmith84@proton.me>

Author: tas50