Allow simplecov 1.x while keeping Ruby 3.1/3.2 installable

[snaka]

Summary

Replaces #764.

#764 bumps simplecov to ~> 1.3, but simplecov 1.3.0+ declares required_ruby_version >= 3.3. Since split still supports Ruby >= 3.1, bundle lock fails on the Ruby 3.1 and 3.2 jobs:

Because simplecov >= 1.3.0 depends on Ruby >= 3.3
  and Gemfile depends on simplecov ~> 1.3,

(The Ruby 3.3+ jobs in #764 were cancelled by fail-fast, not failing on their own.)

This PR widens the requirement to >= 0.15, < 2 instead, so Bundler resolves the newest simplecov compatible with each Ruby. There is no committed Gemfile.lock, so each CI job resolves independently. The spec helper only calls SimpleCov.start, which behaves the same on 0.x and 1.x.

Verification (local)

Ruby	Rails	simplecov resolved	Result
3.1.6	7.1	0.22.0	522 examples, 0 failures
3.2.2	8.1	1.2.0	522 examples, 0 failures
3.3.5	8.1	1.3.1	522 examples, 0 failures
4.0.1	8.1	1.3.1	522 examples, 0 failures

RuboCop: no offenses.

🤖 Generated with Claude Code