Update rustyline requirement from 6 to 8

[dependabot-preview[bot]]

Updates the requirements on rustyline to permit the latest version.

Release notes

Sourced from rustyline's releases.

8.0.0

Breaking Changes

• Fix WordAction variants (https://rust-lang.github.io/rust-clippy/master/index.html#upper_case_acronyms)

Changelog

• Conditional binding (#492, #488, #436, #407, and partially #430, #269)
• Bracketed paste can be disabled (#202)
• Upgrade nix dependency to version 0.20
• Upgrade skim optional dependency to version 0.9

Commits

• b99e014 Merge pull request #502 from gwenn/8.0.0
• c32caac Prepare release of version 8.0.0
• 9c419ba Merge pull request #492 from gwenn/conditional_binding2
• bf55c10 Add example for custom key bindings
• 3d692d3 Add example for custom bindings
• bf5bbfd Add examples for custom key bindings
• 15d9f43 Merge remote-tracking branch 'upstream/master' into conditional_binding2
• 4eeb2e4 Merge pull request #501 from gwenn/cow
• b00e10f Fix warning
• f53c4c1 Merge pull request #499 from gwenn/update_deps
• Additional commits viewable in compare view

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[Johann150]

@dependabot ignore this dependency

[dependabot-preview[bot]]

OK, I won't notify you about rustyline again, unless you re-open this PR or update it yourself. 😢

[Johann150]

This breaks the minimum Rust version in the tests, see also #301