Update syn requirement from 1.0 to 2.0

[dependabot[bot]]

Updates the requirements on syn to permit the latest version.

Release notes

Sourced from syn's releases.

2.0.49

• Improve error location when parsing from an empty string literal using LitStr::parse (#1590)

Commits

• e64c063 Release 2.0.49
• 981359c Merge pull request #1590 from dtolnay/streof
• 51298d4 Improve error location at eof in LitStr::parse
• 270c633 Update test suite to nightly-2024-02-13
• dc9cf16 Remove FilterAttrs trait when unused
• 7dcfac7 Ignore dead_code warning in test
• 9831844 Update signature of Emitter::emit_diagnostic in nightly-2024-02-07
• 9e8033f Update test suite to nightly-2024-02-07
• cb3348c Update test suite to nightly-2024-01-23
• 15b9dbc Update test suite to nightly-2024-01-19
• Additional commits viewable in compare view

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