`VerbAttribute` has unnecessary breaking changes between 2.8.0 and 2.9.1

**Describe the bug**
`VerbAttribute`'s constructor signature has changed between 2.8.0 and 2.9.1. There is a default value for the new alias parameter, so recompilation of the same source code is possible without error. However, upgrading without rebuilding is not possible.

**To Reproduce**
Steps to reproduce the behavior:
1. Create an exe depending on CommandLineParser 2.8.0 that uses `VerbAttribute`
2. Replace CommandLineParser 2.8.0 with CommandLineParser 2.9.1 in the bin folder
3. Execute the exe
4. See error

**Additional context**
When having multiple exes in the same folder, all exes need to be rebuilt and updated simultaneously along with updating CommandLineParser in order to upgrade.

Reintroducing the 2.8.0 `VerbAttribute` constructor, (and possibly labeling it as obsolete), would allow assemblies built with version 2.8.0 and assemblies built with version 2.9.1 to both work with the new version (possibly requiring binding redirects).