Possibility to exclude letter combinations

[knstntnb]

Summary of feature request

As discussed in https://github.com/shlinkio/shlink/discussions/1666 I think an option to exclude specific letter-combination would be very helpful to avoid combinations, like "FCK3v" or "PsY1l", which I got during testing.

Options could be defined via an env for docker, such as defining specific combinations or reference an external file, but also open for discussion and/or ideas.