The documentation page "Moving from 2.4 to 2.5" does not contain information about API changes for Java actions yet. This is an omission, it will be added as soon as possible.
Concerning the examples you gave:
com.mendix.modules.exportmanager.excel.ExcelGridwas never meant to be used in Java actions, but was not obfuscated in 2.4, which is why you could use this class in that version. Only classes in
com.mendix.modules.exportmanager.interfacesshould be used for Excel export. This is enforced in 2.5, which causes the error you describe.
Configurationclass can no longer be accessed statically in 2.5.
Core.getConfiguration()can be used to obtain the
IContextparameter to do security checks.
Hope this helps to solve your errors, thank you for pointing out this omission in the documentation.
For your first problem, try using the IExcelGrid, it can be found in com.mendix.modules.exportmanager.interfaces.excel.
For your second problem, you can obtain the configuration by calling Core.getConfiguration() now.