Move Gitlab::UntrustedRegexp into gitlab-utils

What does this MR do and why?

Gitlab::UntrustedRegexp and Gitlab::UntrustedRegexp::RubySyntax are generic RE2 wrappers with no GitLab concepts. Moving them into gitlab-utils is the first step of extracting the CI config code into a gitlab-ci-config gem, which can then depend on gitlab-utils instead of on re2 and the monolith.

The two feature flag checks in with_fallback become ports on the class, wired in config/initializers/900_gem_gitlab-utils.rb:

• fallback_disabled replaces Feature.enabled?(:disable_unsafe_regexp). It defaults to true, so the gem refuses the unsafe fallback unless the host allows it.
• fallback_logger replaces the ci_unsafe_regexp_logger logging and is called with the class name and the pattern. It defaults to doing nothing.

spec/initializers/gitlab_utils_untrusted_regexp_spec.rb covers the adapters. The malicious regexp shared example moves into gitlab-utils so the gem and the three monolith specs using it share one copy.

gitlab-utils gains a runtime dependency on re2 with the monolith's constraint. The root lockfiles stay on re2 2.27.0. qa/Gemfile.lock now resolves re2, which needs mini_portile2 2.8.9.

Part of #632651.

Context

Part of #635234, which has the plan, verification and risks for this workstream. The tracking issue for the whole slice is #632651.

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