deps: bump regex-syntax minimum version to 0.6.20
deps: bump regex-syntax minimum version to 0.6.20

rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex