fix: add xml 7.x support (#761)

[Akhrameev]

What does this change?

Fixes #761. flutter_gen_core pinned xml: ^6.0.0, so any project depending
on both flutter_gen/flutter_gen_runner and xml: ^7.0.0 failed version
solving.

This widens the constraint to xml: '>=6.0.0 <8.0.0' instead of bumping to
^7.0.0, deliberately:

• Non-breaking. Consumers on xml 7.x (and Dart ≥ 3.11) now resolve cleanly,
  while consumers on older toolchains keep working — pub falls back to xml 6.x
  automatically. A hard ^7.0.0 would instead force everyone onto xml 7,
  image ≥ 4.9.x, and effectively Dart ≥ 3.11 (xml 7 requires sdk: ^3.11.0),
  which would also require bumping this package's own sdk floor.
• Robust. The range gives the resolver a fallback (xml 6.x + image 4.8.x),
  so it stays solvable through transitive image/meta/Flutter-SDK pin shifts
  that a single forced major can't survive.

Also replaces the deprecated XmlNode.text with XmlNode.innerText. This is
cleanup, not required to build: XmlNode.text is only deprecated in xml 7.x
(deprecated starting from 6.3.0), not removed, and the call was already
// ignore-suppressed. innerText has existed since xml 6.0.0, so it's
source-compatible with both majors and future-proofs against an eventual
removal in xml 8.x.

To guard both majors going forward, adds a cross-platform Dart matrix
(scripts/test_xml_versions.dart, exposed as melos test:xml-matrix) that runs
flutter_gen_core's tests against both xml 6.x and 7.x via a temporary
dependency_overrides, plus a CI step that runs it on every PR.

Fixes #761 🎯

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
  • Ensure the tests (melos run test)
  • Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)
• Appropriate docs were updated (if necessary)

[AlexV525]

One thing I want to know about API compatibility

[AlexV525]

Is this a new API introduced in v7? If yes then it's not compatible with v6 then.

[Akhrameev]

No, it appeared earlier. I'll find a proof

[Akhrameev]

it was introduced in 4.2.0 and deprecated in 6.3.0

[Akhrameev]

@AlexV525 earlier my newly added tests failed (not because of code part - but anyway). Please restart workflow to see actual state

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.37%. Comparing base (89c1920) to head (ee80d45).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #764   +/-   ##
=======================================
  Coverage   97.37%   97.37%           
=======================================
  Files          24       24           
  Lines         989      989           
=======================================
  Hits          963      963           
  Misses         26       26           

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