Update syn requirement from 1 to 2

[dependabot[bot]]

Updates the requirements on syn to permit the latest version.

Release notes

Sourced from syn's releases.

2.0.2

• Documentation improvements

Commits

• cac5cc6 Release 2.0.2
• 8f826ef Touch up spacing in example for parse_multi_with_leading_vert
• fc58fcf Fix typo in Stmt::Macro documentation
• e298152 Release 2.0.1
• b87a0a1 Merge pull request #1409 from dtolnay/requiremeta
• 59dd7cc Add methods on Meta for error reporting an incorrect kind of attribute
• 11c0b6c Build attribute parse errors using std::fmt system
• e6cf741 Move parse_args_with error handler into parsing module
• 906fa56 Release 2.0.0
• 29bd855 Fix warning about Cargo.toml exclude/include
• Additional commits viewable in compare view

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