fix: prevent cells shrinking when not using focus button mode (#9913)

Author: web-padawan

packages/grid-pro/src/styles/vaadin-grid-pro-base-styles.js

@@ -23,7 +23,7 @@ const gridPro = css`
     display: flex;
     align-items: inherit;
     align-self: stretch;
-    flex: 1;
+    flex-grow: 1;
     min-width: 0;
     cursor: var(--vaadin-clickable-cursor);
     outline: 0;