German maps: Rename "Lower Saxony" to "Lower Saxony, Bremen"

[scaidermern]

Description

The German map for "Lower Saxony" (Niedersachsen) contains all the details for the state "Bremen". However this is not clear to the user. The user will very likely install both maps (Lower Saxony and Bremen) - resulting in overlapping maps - resulting in strange/wrong behavior (#15039).

For "Brandenburg" and "Berlin" this has been resolved by renaming the map for "Brandenburg" to "Brandenburg, Berlin". This makes it clearer that all the data for "Berlin" is included.

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