Allow newer `unicode-display_width`

[DannyBen]

The gemspec is blocking the latest 3.x version of unicode-display_width

strings/strings.gemspec

Line 28 in 30854c1

spec.add_dependency "unicode-display_width", ">= 1.5", "< 3.0"

As this gem is used by tty-markdown, it is leaving this older gem as the only outdated one.

$ bundle outdated
Fetching gem metadata from https://rubygems.org/............
Resolving dependencies...

Gem                    Current  Latest  Requested  Groups
unicode-display_width  2.6.0    3.2.0

Would be nice if the < 3 restriction was expanded.

I wanted to create a PR, but noticed that the CI needs updates for newer Ruby versions as well, so I backed off.