Context:

With OPT-4212 we are adding distribution by number variables, which includes custom bucket options. For this purpose, an additional `CustomBucketDto` field will be added to `SingleReportConfigurationDto` to specify the custom bucket settings for a distributed by number variable report. 

There is already a `CustomBucketDto` field in `SingleReportConfigurationDto` which is used for groupBy number variable reports. This field is currently named `customBucket`. To avoid confusion, this field needs to be renamed, for example to something like `groupByNumberVariableCustomBucket`. Existing reports need migrated.

Note that the frontend will need adjusted as well.

AT:

• SingleReportConfigurationDto.customBucket field is renamed to make it clear that its used for groupBy number variable reports and differentiate from the field used for the distributedBy option
• existing reports are migrated
• confluence documentation is updated