Deal with "CA2213: Disposable fields should be disposed" violations

We turned this rule off here: https://github.com/dotnet/project-system/pull/3926, but there are some instances that were suppressed that are actual issues, where we're not disposing the field. This tracks fixing them:

- [ ] https://github.com/dotnet/project-system/blob/9be23089d1c384b18fcad50820c93e6ec52b2952/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/PropertyPages/WpfBasedPropertyPage.cs#L11
- [ ] https://github.com/dotnet/project-system/blob/f28a7397ca13a57e08fb9db9e7dcbe35eb977c8b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Threading/Tasks/SequencialTaskExecutor.cs#L19
- [ ] https://github.com/dotnet/project-system/blob/dfe534a3e7803efb763ba9dcd35c5ce60c9104a5/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/Subscriptions/DependencySharedProjectsSubscriber.cs#L24
- [ ] https://github.com/dotnet/project-system/blob/dfe534a3e7803efb763ba9dcd35c5ce60c9104a5/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/CrossTarget/CrossTargetRuleSubscriberBase.cs#L19
- [ ] https://github.com/dotnet/project-system/blob/db9429c4e7a7fa405f067e7e612d1a4a71bb32ef/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/CrossTarget/CrossTargetSubscriptionHostBase.cs#L18
- [ ] https://github.com/dotnet/project-system/blob/db9429c4e7a7fa405f067e7e612d1a4a71bb32ef/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/LanguageServices/LanguageServiceHost.cs#L23