Airports; ICAO codes cannot have digits

Hey all,

Love this library and will provide a PR for my native locale `Sv` pretty soon 🙂

I used `Faker.Airports.icao/0` in my test suite and the test for ICAO codes failed. Looking at the source code I see that it for instance lists [`19AK`](https://github.com/elixirs/faker/blob/1f42d2bf89f66214270804196b8863c860237518/lib/faker/airports.ex#L163C6-L163C10) for [Icy Bay Airport](https://en.wikipedia.org/wiki/Icy_Bay_Airport). That air port has no ICAO code assigned to it according to the Wikipedia page, but it does have an FAA location indicator (LID) `19AK`.

The most recent ICAO document 7910/158 available to me lists only letters. I'm afraid that's one of the pay for copies standards.

Not knowing how you would like to handle it I haven't yet provided a PR but I would be happy to do so with your input.

There are datasets such as [OurAirports](https://ourairports.com/data/) that could be useful to sample.

Thanks again for this library 👋🏻

Start in lib/faker/airports.ex at the Icy Bay Airport entry used by Faker.Airports.icao/0, and compare that value with an authoritative ICAO listing rather than the FAA location indicator. Clarify how airports without ICAO assignments should be handled, update the airport data accordingly, and verify that generated ICAO codes no longer include invalid numeric entries.

Written by the indexing model from the issue text.