build: bump syn from 2.0.67 to 2.0.68 #26389

Job	Run time
Rust - test_postgres	8m 50s
Rust - ockam_command_cross_build (linux_armv7)	13m 2s
Rust - lint_cargo_readme	1m 31s
Rust - ockam_command_cross_build (macos_silicon)	8m 34s
Rust - check_cargo_update	4m 9s
Rust - lint_cargo_fmt_check	1m 30s
Rust - test	17m 42s
Rust - check_no_std	4m 8s
Rust - lint_cargo_clippy	4m 56s
Rust - test_nightly	18m 21s
Rust - check_nightly	4m 14s
Rust - test_ockam_command (linux_86)	7m 22s
Rust - lint_cargo_deny	1m 36s
Rust - lint_cargo_toml_files	1m 35s
Rust - lint_cargo_machete	1m 29s
Rust - build	5m 21s
Rust - build_docs	4m 18s
Rust - build_examples	5m 26s
Rust - build_nightly	5m 44s
Rust - build_release	14m 7s
	2h 13m 55s

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