depends: Update Qt to 6.8.4 #35709

Release notes: https://code.qt.io/cgit/qt/qtreleasenotes.git/about/qt/6.8.4/release-note.md

depends/patches/qt/qtbase_platformsupport.patch has been dropped as the fix was backported upstream in https://github.com/qt/qtbase/commit/5afcc64fd774ba7337b688841a852d1a1a242554.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35709.

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Hmm, observing the following build error:

/gnu/store/r03804zpq5i6wsalx0yaqrr5jb7pqrmv-binutils-cross-riscv64-linux-gnu-2.46.0/bin/riscv64-linux-gnu-ld: /bitcoin/depends/riscv64-linux-gnu/lib/libQt6BundledLibpng.a(pngrutil.c.o): in function `png_read_filter_row':                              
pngrutil.c:(.text.png_read_filter_row+0x8c): undefined reference to `png_init_filter_functions_rvv'                                                                                                                                                       
/gnu/store/r03804zpq5i6wsalx0yaqrr5jb7pqrmv-binutils-cross-riscv64-linux-gnu-2.46.0/bin/riscv64-linux-gnu-ld: relocation R_RISCV_CALL_PLT against `png_init_filter_functions_rvv' which may bind externally can not be used when making a shared object; r
ecompile with -fPIC