[versions] update to the latest sdk versions

[jonahwilliams]

Description

Update to more null-safety versions

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[zanderso]

rslgtm

[christopherfujino]

LGTM what could go wrong?

[christopherfujino]

LGTM what could go wrong?

other than all the tool_tests failing...

[christopherfujino]

https://cirrus-ci.com/task/5116457497395200?command=main#L2139

ERR : Because test >=1.15.3 <1.16.0-nullsafety depends on test_api 0.2.18 and every version of flutter_driver from sdk depends on test_api 0.2.19-nullsafety, test >=1.15.3 <1.16.0-nullsafety is incompatible with flutter_driver from sdk.
    | So, because flutter_project depends on both flutter_driver any from sdk and test 1.15.3, version solving failed.
FINE: Exception type: SolveFailure
FINE: package:pub/src/solver/version_solver.dart 312:5   VersionSolver._resolveConflict
    | package:pub/src/solver/version_solver.dart 133:27  VersionSolver._propagate
    | package:pub/src/solver/version_solver.dart 97:11   VersionSolver.solve.<fn>
    | ===== asynchronous gap ===========================
    | dart:async                                         Future.catchError
    | package:pub/src/utils.dart 113:52                  captureErrors.wrappedCallback
    | package:stack_trace                                Chain.capture
    | package:pub/src/utils.dart 126:11                  captureErrors
    | package:pub/src/command_runner.dart 193:13         PubCommandRunner.runCommand

[christopherfujino]

Possibly #61687