Update syn requirement from 1.0.5 to 2.0.18

[dependabot[bot]]

Updates the requirements on syn to permit the latest version.

Release notes

Sourced from syn's releases.

2.0.18

• Permit empty attr in syn::meta::parser (#1460)

Commits

• 4cae0a2 Release 2.0.18
• 278dbe1 Merge pull request #1461 from dtolnay/emptyattr
• f1ff0e9 Permit empty attr in syn::meta::parser
• c59a68d Release 2.0.17
• c49115a Eliminate pm rename for proc_macro crate
• ab19415 Merge pull request #1459 from dtolnay/wasm
• ee29399 Extern crate proc_macro available on all wasm targets
• 4e97063 Show error details during miri setup in CI
• 9aa5c63 Release 2.0.16
• efda52a Merge pull request #1454 from dtolnay/builtin
• Additional commits viewable in compare view

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