chore: Update appium_lib_core requirement from ~> 6.0 to >= 6, < 8 (#991)

Updates the requirements on [appium_lib_core](https://github.com/appium/ruby_lib_core) to permit the latest version.
- [Changelog](https://github.com/appium/ruby_lib_core/blob/master/CHANGELOG.md)
- [Commits](appium/ruby_lib_core@v6.0.0...v7.0.0)

---
updated-dependencies:
- dependency-name: appium_lib_core
  dependency-type: direct:production
...

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

Author: dependabot[bot]

appium_lib.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
   s.homepage      = 'https://github.com/appium/ruby_lib' # published as appium_lib
   s.require_paths = ['lib']
 
-  s.add_runtime_dependency 'appium_lib_core', '~> 6.0'
+  s.add_runtime_dependency 'appium_lib_core', '>= 6', '< 8'
   s.add_runtime_dependency 'nokogiri', '~> 1.8', '>= 1.8.1'
   s.add_runtime_dependency 'tomlrb', '>= 1.1', '< 3.0'