Improve attribute handling for elements that are safe

[KirkMunroSagent]

📝 Provide a description of the improvement

Today in CKEditor5, if you include <br> elements inside of a paragraph when your editor does not have the ShiftEnter plugin loaded, those elements are marked as unsafe using attribute prefixing (per PR #10801). This is misleading to CKEditor users, because it sends developers off in the wrong direction when trying to figure out why their <br> elements are not rendering properly. A <br> element is always safe. Instead of marking these elements as unsafe when the plugin required to render them is not loaded, perhaps you could consider using a prefix that indicates that the element does not render because the plugin required to do so is not loaded? That would be more actionable and more appropriate than marking the element as unsafe.

📃 Other details

• Browser: Microsoft Edge, latest version.
• OS: Windows 11
• CKEditor version: 43.1.0
• Installed CKEditor plugins:
  CKEditor5.alignment.Alignment,
  CKEditor5.basicStyles.Bold,
  CKEditor5.basicStyles.Italic,
  CKEditor5.basicStyles.Subscript,
  CKEditor5.basicStyles.Superscript,
  CKEditor5.basicStyles.Underline,
  CKEditor5.font.Font,
  CKEditor5.heading.Heading,
  CKEditor5.horizontalLine.HorizontalLine,
  CKEditor5.htmlSupport.GeneralHtmlSupport,
  CKEditor5.image.Image,
  CKEditor5.image.ImageInsertViaUrl,
  CKEditor5.image.ImageStyle,
  CKEditor5.image.ImageToolbar,
  CKEditor5.link.Link,
  CKEditor5.list.List,
  CKEditor5.showBlocks.ShowBlocks,
  CKEditor5.sourceEditing.SourceEditing,
  CKEditor5.table.Table,
  CKEditor5.table.TableCellProperties,
  CKEditor5.table.TableColumnResize,
  CKEditor5.table.TableProperties,
  CKEditor5.table.TableSelection,
  CKEditor5.table.TableToolbar,
  CKEditor5.undo.Undo,

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