[go_router_builder] Allow build 5 - #13455

Widens the build constraint to ">=3.0.0 <6.0.0" so apps can use build_runner 2.17.1, which requires build 5.

build 5.0.0 republishes the retracted 4.1.0 as a major version. Its only breaking change is the new BuildStep.librarySourceSink member, which breaks packages that implement BuildStep. go_router_builder only receives a BuildStep in GoRouterGenerator.generate and does not implement it, so no code changes are needed.

This also removes the TODO(piinks): Pin version once new stable rolls. above the constraint. It was added in #10078 because the Flutter stable of the time pinned a test version that could not resolve build 4, so the range had to include build 3; the plan was to switch to ^4.0.0 once a newer stable shipped. The range is now meant to span build 4 and 5, and pinning to ^5.0.0 would force build_runner 2.17.1 and analyzer 14.3 on every user, so the TODO no longer applies. Happy to restore it if you'd rather keep it.

Tested locally on Flutter 3.47.8 (Dart 3.13.7), which resolves build 5.0.0, build_test 3.5.23, build_runner 2.17.1, source_gen 4.3.1 and analyzer 14.6.0:

• custom-test --packages=go_router_builder (the tool/run_tests.dart builder golden tests): all 56 pass.
• dart-test --packages=go_router_builder --platform=vm: all pass (10 in the package, 19 in the example). On stable the example still resolves build 4.0.11 and build_runner 2.16.1, because the test_api pinned by flutter_test caps analyzer below 14.
• analyze --packages=go_router_builder: no issues. analyze --downgrade resolves build 4.0.4 and also reports no issues.

Test exemption: this is a manual dependency roll that only widens a constraint. The existing builder tests cover generation with build 5 resolved.

Fixes flutter/flutter#194542

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