[8.x] Mark MaintenanceModeException as deprecated

[vigneshgurusamy]

Ref: #35635

[driesvints]

Please explain why this is needed and don't link to an issue, thanks.

[vigneshgurusamy]

As I mentioned in the issue, Taylor has migrated the Maintenance mode exception from MaintenanceModeException to HttpException to enhance the feature and it was not updated in the documentation.

For two reasons I have created this PR:

1. MaintenanceModeException is not used anywhere in the framework code.
2. More than one person was affected by this change. So by deprecating the unused exception will help the people to identify the change without BC and IDE will highlight the deprecation if it is used in their code.