The ornamental design for a retainer, as shown and described.
FIG. 1 is a bottom perspective view of a retainer depicting a new design;
FIG. 2 is a top perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a back elevational view thereof;
FIG. 5 is a left elevational view thereof;
FIG. 6 is a right elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a second bottom perspective view thereof.
The broken line disclosures depicting a lighting fixture in FIG. 9 represent environmental structure that forms no part of the claimed design. The remaining broken line disclosures in the views represent portions of the retainer that form no part of the claimed design.