refactor: use Base64 instead of Base64Utils

[NaccOll]

SpringBoot 3.4 will remove Base64Utils

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.31%. Comparing base (4f9d0ae) to head (1eeb7cd). Report is 1831 commits behind head on master.

Files with missing lines	Patch %	Lines
...server/web/client/BasicAuthHttpHeaderProvider.java	60.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3858 +/- ## ============================================ - Coverage 83.64% 82.31% -1.33% + Complexity 1247 1222 -25 ============================================ Files 156 162 +6 Lines 3644 4095 +451 Branches 258 261 +3 ============================================ + Hits 3048 3371 +323 - Misses 464 603 +139 + Partials 132 121 -11 ```

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