Progress: add strokeLinecap prop (#17552)

* Progress: add strokeLinecap prop

* update docs

types/progress.d.ts

@@ -14,6 +14,9 @@ export declare class ElProgress extends ElementUIComponent {
   /** The width of progress bar */
   strokeWidth: number
 
+  /** Circle progress bar stroke line cap */
+  strokeLinecap: string
+
   /** Whether to place the percentage inside progress bar, only works when type is 'line' */
   textInside: boolean