Support Elixir 1.18 in GitHub CI

[kianmeng]

List of changes:

• resolve type warnings in test case
• use specific versions by string instead of number in CI matrix, see https://github.com/erlef/setup-beam?tab=readme-ov-file#specify-versions-as-strings-not-numbers

[kianmeng]

Run mix test
  
    warning: incompatible types given to Kernel.elem/2:
        elem(__CALLER__.function, 0)
    given types:
        -nil or {atom(), integer()}-, integer()
    but expected one of:
        {...}, integer()
    typing violation found at:
    │
 21 │     path = Path.join([tmp_path(), "#{__CALLER__.module}", "#{elem(__CALLER__.function, 0)}"])
    │                                                              ~
    │
    └─ test/support/file_helpers.ex:[21](https://github.com/kianmeng/ex_aws_s3/pull/1/checks#step:9:22):62: Support.FileHelpers.in_tmp/1