deps/bitsery: Use lower-case only in FetchContent to fix silently failing fetching of bitsery.

[MisterGC]

At least when using emscripten based cross-compilation for erdblick, the bitsery dependency was not fetched properly. There was not source/build dir in the _deps dir. Not sure if it was hidden somewhere else but at the end this caused a linker error, because of the non-present bitsery library. This PR just changes to all-lower-case. I think it is somehow related to the way cmake populates variables when using FetchContent, but I don't know for sure. Nevertheless, I guess this should not have any negative side-effect but makes the cross-compilation work.

[sonarcloud[bot]]

Quality Gate passed

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[github-actions[bot]]

Package	Line Rate	Branch Rate	Health
include.simfil	61%	42%	➖
include.simfil.model	82%	59%	✔
src	63%	36%	➖
src.model	79%	44%	➖
Summary	68% (2497 / 3684)	38% (2042 / 5335)	➖

Minimum allowed line rate is 50%

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Test Results

 1 files  ±0   1 suites  ±0   3m 5s :stopwatch: -1s 40 tests ±0  40 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  41 runs  ±0  41 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 8fe9e5bd. ± Comparison against base commit 200750a7.