Update regex-syntax requirement from 0.7 to 0.8 (#386)

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-0.7.0...regex-syntax-0.8.2)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

proptest/Cargo.toml

@@ -75,7 +75,7 @@ features = ["libm"]
 [dependencies.regex-syntax]
 # If you change this, make sure to also bump the `regex` dependency to a
 # version that also uses this version of regex-syntax.
-version = "0.7"
+version = "0.8"
 optional = true
 
 [dependencies.bit-set]