Adapt to FindBoost deprecation with CMake 3.30

[aurianer]

Fixes #1222

We require Boost version >= 1.71 in pika, BoostConfig.cmake is available in boost installations since 1.70, so only changing module to config was necessary.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.02% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (89f5337)	18360	13718	74.72%
Head commit (236e3a1)	18358 (-2)	13713 (-5)	74.70% (-0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1229)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[msimberg]

Thanks! We'll need a newer commit of spack which contains spack/spack#46062, but otherwise this is ok to merge.

[msimberg]

There seems to be a concretization issue on the nvhpc pipeline with the new spack version: https://gitlab.com/cscs-ci/ci-testing/webhook-ci/mirrors/479009878135925/5304355110917878/-/jobs/7742931167. I'm going to risk merging this to get the next release out with the assumption that the error is simply some overconstraint in spack and that there's no issue in the pika build with that pipeline once we get it working.

[msimberg]

The clang pipeline failure also needs spack/spack#46200.