checker: replace warning by notice for UTF8 strings validation

[lcheylus]

Fix #24538

---

Tests OK on Linux (Debian/amd64)

$ ./v -stats -W test vlib/encoding/utf8/validate/encoding_utf8_test.v
---- Testing... ----------------------------------------------------------------------------------------------------------------------------
vlib/encoding/utf8/validate/encoding_utf8_test.v:6:36: notice: invalid utf8 string, please check your file's encoding is utf8
    4 |     assert validate.utf8_string('añçá') == true
    5 |     assert validate.utf8_string('\x61\xC3\xB1\xC3\xA7\xC3\xA1') == true
    6 |     assert validate.utf8_string('\xC0\xC1') == false
      |                                       ~~~~
    7 |     assert validate.utf8_string('\xF5\xFF') == false
    8 |     assert validate.utf8_string('\xE0\xEF') == false
vlib/encoding/utf8/validate/encoding_utf8_test.v:7:36: notice: invalid utf8 string, please check your file's encoding is utf8
    5 |     assert validate.utf8_string('\x61\xC3\xB1\xC3\xA7\xC3\xA1') == true
    6 |     assert validate.utf8_string('\xC0\xC1') == false
    7 |     assert validate.utf8_string('\xF5\xFF') == false
      |                                       ~~~~
    8 |     assert validate.utf8_string('\xE0\xEF') == false
    9 | }
vlib/encoding/utf8/validate/encoding_utf8_test.v:8:36: notice: invalid utf8 string, please check your file's encoding is utf8
    6 |     assert validate.utf8_string('\xC0\xC1') == false
    7 |     assert validate.utf8_string('\xF5\xFF') == false
    8 |     assert validate.utf8_string('\xE0\xEF') == false
      |                                       ~~~~
    9 | }
checker summary: 0 V errors, 0 V warnings, 3 V notices
        V  source  code size:      29945 lines,     137986 tokens,     804318 bytes,   286 types,    13 modules,   134 files
generated  target  code size:       9980 lines,     352219 bytes
compilation took: 338.826 ms, compilation speed: 88378 vlines/s, cgen threads: 7
running tests in: /home/fox/dev/vlang.git/vlib/encoding/utf8/validate/encoding_utf8_test.v
      OK       0.005 ms     5 asserts | main.test_validate_str()
     Summary for running V tests in "encoding_utf8_test.v": 5 passed, 5 total. Elapsed time: 0 ms.

 OK     358.708 ms vlib/encoding/utf8/validate/encoding_utf8_test.v
--------------------------------------------------------------------------------------------------------------------------------------------
Summary for all V _test.v files: 1 passed, 1 total. Elapsed time: 360 ms, on 1 job. Comptime: 0 ms. Runtime: 358 ms.

[huly-for-github]

Connected to Huly®: V_0.6-22910

[lcheylus]

Force new push => forgot to modify output for vlib/v/checker/tests/invalid_utf8_string.vv test used by vlib/v/compiler_errors_test.v

[spytheman]

Excellent work. A note is indeed better than a warning for this.