docs: add changelog item on dropped classes and methods

[DeepDiver1975]

Description

Change: Removed legacy and deprecated code from ownCloud 11.

The following have been removed:

• class OC_DB
• class OC_DB_StatementWrapper
• class OC_Group_Backend
• class OC_Group_Database
• class OC_OCS_Result
• class \OCP\DB
• class MDBSchemaWriter
• interface OC_Group_Interface
• interface OC_User_Interface
• method MDB2SchemaManager::getDbStructure()
• method MDB2SchemaManager::generateChangeScript()

#41455
#41458
#41462
#41464
#41468

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE