Update rustfft requirement from ^5.0 to ^6.1

[dependabot[bot]]

Updates the requirements on rustfft to permit the latest version.

Release notes

Sourced from rustfft's releases.

6.1.0

[6.1]

Released 7th Novemeber 2022

Added

• Implemented a code path for Neon-optimized FFTs on AArch64 (Thanks to Henrik Enquist!) (#84 and #78)

Changed

• Improved performance of power-of-3 FFTs when not using SIMD-accelerated code paths (#80)
• Reduced memory usage for some FFT sizes (#81)

Commits

• c55e8f2 Changes for 6.1.0 (#97)
• 7bc45c9 Reorganize readme (#96)
• 0a7ad8a Support for neon on stable rust (#84)
• b0d9bd0 Save memory by skipping the shuffle map from Radix4 and Radix3 (#81)
• 64d1274 Added butterfly9 and butterfly27, and Radix3 (#80)
• 3dd012c Neon (#78)
• f80d7dd Add v5.1.1 to changelog
• 66c2e01 Version 6.0.1 (#76)
• 0139fd3 fix constants for imm2 and imm4 (#75)
• 5ff1fa2 Update Cargo.toml (#71)
• Additional commits viewable in compare view

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[dependabot[bot]]

Looks like rustfft is up-to-date now, so this is no longer needed.