Simplify bin/bump-version

[jeffwidman]

Over in
#360 (comment), I noticed that bash was complaining about this script:

bin/bump-version: line 9: ((: patch_level == 'major' || patch_level == 'minor' || patch_level == 'patch': syntax error: operand expected (error token is "'major' || patch_level == 'minor' || patch_level == 'patch'")

I started to dig into it, but the while loop isn't needed, the case statement felt unecessarily complex so I simplified it to use an if statement.

I also changed the argument from a flag-based argument to simple ordered argument, as again it seemed simpler and it matches the style of the bump version script over in dependabot-core so it's easier for engineers working across repos. If we later have additional flags, we can always switch it back later.

Lastly, I found patch_version confusing given that patch is a specific value that can be used, so I renamed it to version_type.

[brrygrdn]

👍🏻