Eigen: backport DisableStupidWarnings.h, clearer license expression

[valgur]

Summary

Changes to recipe: eigen/[*]

Motivation

Eigen is in no rush to release a new version and the existing versions produce an insane amount of irrelevant warnings on nvcc.

Details

• Copy the current latest version of DisableStupidWarnings.h. This matches the behavior of Vcpkg, for example: https://github.com/microsoft/vcpkg/blob/master/ports/eigen3/update-warning-suppression-to-latest.patch
• Also made the license expression clearer. See https://repology.org/project/eigen/information for the expressions used by other package managers.
• Cleaned up some Conan v1 cruft.

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• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan