bump to 1.6.0

Because why the hell not?

1.6.0 (2022-07-05)
New features:
    FEATURE clap-rs#832: Clarify that Captures::len includes all groups, not just matching groups.
    FEATURE clap-rs#857: Add an ExactSizeIterator impl for SubCaptureMatches.
    FEATURE clap-rs#861: Improve RegexSet documentation examples.
    FEATURE clap-rs#877: Upgrade to Unicode 14.
Bug fixes:
    BUG clap-rs#792: Fix error message rendering bug.

1.5.6 (2022-05-20)
    BUG clap-rs#680: Fixes a bug where [[:alnum:][:^ascii:]] dropped [:alnum:] from the class.
    BUG clap-rs#859: Fixes a bug where Hir::is_match_empty returned false for \b.
    BUG clap-rs#862: Fixes a bug where 'ab??' matches 'ab' instead of 'a' in 'ab'.

Cargo.toml

@@ -129,11 +129,11 @@ atty = { version = "0.2",  optional = true }
 termcolor = { version = "1.1.1", optional = true }
 terminal_size = { version = "0.1.12", optional = true }
 lazy_static = { version = "1", optional = true }
-regex = { version = "1.5.5", optional = true }
+regex = { version = "1.6", optional = true }
 backtrace = { version = "0.3", optional = true }
 
 [dev-dependencies]
-regex = "1.5.5"
+regex = "1.6"
 lazy_static = "1"
 criterion = "0.3.2"
 trybuild = "1.0.18"