Remove documentation for ApplicationContext obsoletion

[schambers]

ApplicationContext is now obsoleted as per 1777 in FM repo: fluentmigrator/fluentmigrator#1777

We should clean up and remove any documentation that refers to ApplicationContext

• https://fluentmigrator.github.io/articles/obsolete/app-context.html?q=applicationcontext
• https://fluentmigrator.github.io/api/v2.x/FluentMigrator.Runner.VersionTableInfo.IVersionTableMetaData.html?q=applicationcontext
• https://fluentmigrator.github.io/api/v5.x/FluentMigrator.MigrationBase.html#FluentMigrator_MigrationBase_ApplicationContext

Along with any other locations in the documentation that refers to this.