Use caret (^) as the default required_version comparator

[wesleymatosdev]

Closes #6729

Summary

required_version currently treats a bare version (no comparator prefix) as
an exact match, overriding the semver crate's own default of a caret
requirement. As discussed in #6729, this diverges from how Cargo treats
dependency version requirements with no operator, where a bare version like
"1.2.3" is equivalent to "^1.2.3".

This removes the override in check_semver_version so a bare version behaves
like ^version, matching Cargo's convention. The old exact-match behavior is
still available by opting in explicitly with =, e.g.
required_version = "=1.2.3".

• src/config/mod.rs: remove the per-comparator override that forced a bare
  (no-prefix) comparator from semver's own default (Op::Caret) back to
  Op::Exact.
• Configurations.md: documents the new default, adds an explicit "Match on
  exact version" example using =, and notes the caret-default in the
  Possible values bullet.
• CHANGELOG.md: adds a Changed entry under [Unreleased] referencing
  Use caret (^) comparison instead of == for required_version #6729.

Test plan

• cargo test --lib required_version — all 19 existing tests pass unchanged;
  none of them depended on the old exact-only default (they all either use
  the current version as its own requirement, an explicit operator, or a
  requirement whose major/minor already differs enough that caret vs. exact
  makes no difference).
• cargo test --lib check_semver_version — 27 tests pass, including two new
  ones: test_default_caret_match (renamed from test_exact_version_match,
  with the two assertions that change under the new default flipped) and
  test_explicit_exact_match (covers the opt-in = exact-match path).
• cargo test --lib (full suite, binary built first): 235 passed; 0 failed.

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[matthewhughes934]

functionally looks good, I had some thoughts on some of the comments.

View changes since this review

[wesleymatosdev]

suggestions applied. Thank you for reviewing it @matthewhughes934

[ytmimi]

@wesleymatosdev Please rebase these changes on the latest main when you get a chance.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ytmimi]

@wesleymatosdev Please remove Closes #6729 from 08fe6dc's commit message.

[wesleymatosdev]

@ytmimi removed!

[ytmimi]

Thank you!