fix: enhance lax mode to support oldstable and older versions

[k1LoW]

This pull request updates the way the tool checks Go version compatibility, especially in "lax" mode, to make the logic clearer and more accurate. The changes improve both the user documentation and the code, and expand the test coverage to ensure correct behavior.

Documentation and User Experience Improvements:

• Updated the Go version compatibility table in README.md to show both "stable" and "oldstable" versions and clarify how checks behave in different modes.
• Improved the help text for the --lax flag in cmd/root.go to better explain that errors only occur on the stable version, and oldstable or older versions are allowed.

Logic and Test Coverage Enhancements:

• Refined the "lax" mode logic in mod/mod.go to only error if the go.mod directive matches the stable version, allowing oldstable or older versions.
• Expanded test cases in mod/mod_test.go to cover more scenarios, including stable, oldstable, oldstable minor, and older versions in both strict and lax modes.