Update regex-syntax requirement from 0.6.26 to 0.8.4

Updates the requirements on [regex-syntax](https://github.com/rust-lang/regex) to permit the latest version.
- [Release notes](https://github.com/rust-lang/regex/releases)
- [Changelog](https://github.com/rust-lang/regex/blob/master/CHANGELOG.md)
- [Commits](rust-lang/regex@regex-syntax-0.6.26...regex-syntax-0.8.4)

---
updated-dependencies:
- dependency-name: regex-syntax
  dependency-type: direct:production
...

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

api_gen/Cargo.toml

@@ -8,4 +8,4 @@ publish = false
 # Version-locked to not make the CI go out of date
 bindgen = "=0.69.4"
 # Force a higher version to fix https://github.com/rust-lang/regex/issues/770 in MSRV
-regex-syntax = "0.6.26"
+regex-syntax = "0.8.4"