Adjust all concepts to use charlist sigils

[angelikatyborska]

Elixir 1.15 introduces those changes: https://github.com/elixir-lang/elixir/releases/tag/v1.15.0

[Code] Code.format_string!/2 now converts 'charlists' into ~c"charlists" by default
[Inspect] Inspect now renders 'charlists' as ~c"charlists" by default

The official guides got updated charlist syntax too: https://elixir-lang.org/getting-started/binaries-strings-and-char-lists.html#charlists

We should update all code examples in concept documents to also use the sigil as default and just mention the "old" single quote syntax somewhere.