Fix regexp change in OTP 28 that deprecated regexp module attributes

[bjufre]

See elixir-lang/elixir#14381 for details.

With Elixir 1.19, this fixes the following kind of warnings:

==> bankster
Compiling 3 files (.ex)
    warning: storing and reading regexes from module attributes is deprecated, inline the regex inside the function definition instead
    │
 19 │     do: Regex.match?(@bic_validation_regex, String.replace(bic, ~r/\s*/, ""))
    │     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/bankster/bic.ex:19: Bankster.Bic.valid?/1

[railsmechanic]

Nice, thank you for your pull request.