Update to work with Vlang 0.4.9

[kahgoh]

The commits in this PR fixes a build failure and a warning in the CI https://github.com/exercism/vlang/actions/runs/12798355176/job/35682210581?pr=236.

More specifically, the error is:

 ---- Testing... ----------------------------------------------------------------
Error: temp/run_test.v:34:44: error: array_init: no type specified (maybe: `[]Type{}` instead of `[]`)
   32 | 
   33 | fn test_concat_list_of_nested_lists() {
   34 |     assert concat[[]int]([[[1], [2]], [[3]], [[]], [[4, 5, 6]]]) == [
      |                                               ~~~
   35 |         [1],
   36 |         [2],
Error: temp/run_test.v:34:44: error: invalid void array element type
   32 | 
   33 | fn test_concat_list_of_nested_lists() {
   34 |     assert concat[[]int]([[[1], [2]], [[3]], [[]], [[4, 5, 6]]]) == [
      |                                               ~~~
   35 |         [1],
   36 |         [2],
Error: temp/run_test.v:34:43: error: invalid array element: expected `[][]int`, not `[]void`
   32 | 
   33 | fn test_concat_list_of_nested_lists() {
   34 |     assert concat[[]int]([[[1], [2]], [[3]], [[]], [[4, 5, 6]]]) == [
      |                                              ~~~~
   35 |         [1],
   36 |         [2],
checker summary: 3 V errors, 0 V warnings, 0 V notices

and the warning is:

Warning: temp/parallel_letter_frequency.v:9:13: warning: function `encoding.utf8.get_uchar` has been deprecated since 2024-11-17, it will be an error after 2025-05-16; use `.get_rune(s string, index int)` instead
--------------------------------------------------------------------------------
    7 |     lower := utf8.to_lower(word)
    8 |     for r in 0 .. utf8.len(lower) {
    9 |         c := utf8.get_uchar(lower, r)
      |                   ~~~~~~~~~~~~~~~~~~~
   10 |         if utf8.is_letter(rune(c)) {
   11 |             result[c]++

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

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