Bump syn from 1.0.76 to 1.0.77 (#3932)

Bumps [syn](https://github.com/dtolnay/syn) from 1.0.76 to 1.0.77.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](https://github.com/dtolnay/syn/compare/1.0.76...1.0.77)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
17 jobs for master in 22 minutes and 59 seconds (queued for 6 seconds)
Name Stage Failure
failed
generate-impl-guide Build
    = hint: declare the link's URL. For example: `[#vote-recovery]: http://example.com/`

[2021-09-26T01:22:12Z INFO mdbook_linkcheck] 1 broken links found
Error: One or more incorrect links
2021-09-26 01:22:12 [ERROR] (mdbook::renderer): Renderer exited with non-zero return code.
2021-09-26 01:22:12 [ERROR] (mdbook::utils): Error: Rendering failed
2021-09-26 01:22:12 [ERROR] (mdbook::utils): Caused By: The "linkcheck" renderer failed
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
check-transaction-versions Build
2021-09-26 01:23:13 👶 Creating empty BABE epoch changes on what appears to be first startup.    
2021-09-26 01:23:13 🏷 Local node identity is: 12D3KooWEcueac6kRgDVdzYyEXbPLCxHMeskpjrAZWXe8N26hfT7
2021-09-26 01:23:13 📦 Highest known block at #0
2021-09-26 01:23:13 〽️ Prometheus exporter started at 127.0.0.1:9615
2021-09-26 01:23:13 Listening for new connections on 127.0.0.1:9945.
2021-09-26 01:23:18 💤 Idle (0 peers), best: #0 (0x9170…8589), finalized #0 (0x9170…8589), ⬇ 0 ⬆ 0
[!] Both nodes not listening after 5 seconds. Exiting
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
test-build-linux-stable Test
test result: FAILED. 42 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.16s

error: test failed, to rerun pass '-p polkadot-node-core-chain-selection --lib'

real 18m12.168s
user 137m42.823s
sys 5m43.502s
Cleaning up file based variables
ERROR: Job failed: exit code 1