[REF] Use built in functorch, use BackPACK 1.6

[f-dangel]

Updates the dependencies such that BackPACK with PyTorch>=2 is used.
Refactors the functorch utilities to use the new API from torch.func.
Removes dependency from separate functorch package.

[coveralls]

Pull Request Test Coverage Report for Build 5600498494

• 36 of 41 (87.8%) changed or added relevant lines in 1 file are covered.
• 39 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-6.2%) to 86.644%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
curvlinops/examples/functorch.py	36	41	87.8%

Files with Coverage Reduction	New Missed Lines	%
curvlinops/examples/functorch.py	1	88.75%
curvlinops/examples/utils.py	4	50.0%
curvlinops/fisher.py	34	28.85%

Totals
Change from base Build 5600122843:	-6.2%
Covered Lines:	519
Relevant Lines:	599

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💛 - Coveralls