Use pika 0.31.0 in CI

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For debugging debug bloat. Not for merging.

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Bisecting pika it actually seems like pika-org/pika#1295 is the first PR that starts causing issues on the CUDA build (https://gitlab.com/cscs-ci/ci-testing/webhook-ci/mirrors/4700071344751697/7514005670787789/-/jobs/8886005663):

src/CMakeFiles/dlaf.core_object.dir/blas/scal.cpp.o:(.debug_info+0x62d4): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/common/single_threaded_blas.cpp.o:(.debug_info+0x5256): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_impl.cpp.o:(.debug_info+0x6661): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator.cpp.o:(.debug_info+0x3d62): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x482c2): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x489fb): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x49c69): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x4a017): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x4a037): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x58b43): relocation truncated to fit: R_X86_64_32 against `.debug_str'
src/CMakeFiles/dlaf.core_object.dir/communication/communicator_grid.cpp.o:(.debug_info+0x5b43d): additional relocation overflows omitted from the output
collect2: error: ld returned 1 exit status

which is very unfortunate...

While checking for possible culprits I also found that pika-org/pika#1321 (slightly) and pika-org/pika#1346 (significantly) increase debug symbol size. Those were my first suspects, but they were merged after pika-org/pika#1295. pika-org/pika#1321 is a must, so we can't really undo the debug bloat from that one directly, but pika-org/pika#1321 we can slightly undo adding back some more type-erasure.

I will continue investigating if we can undo some changes from pika-org/pika#1295 without going completely back to tag_invoke for set_value.