fix(components): [el-dialog] don't ignore z-index

[purepear]

This is a regression.
El-dialog is ignoring z-index(which by default is incrementally autogenerated).

For example when we have el-dialog nested in el-drawer(even if they are appended directly to the body), the drawer ends up on top because it has z-index while el-dialogs has none.

Repro:
https://element-plus.run/#eyJBcHAudnVlIjoiPHRlbXBsYXRlPlxuXHQ8ZGl2IGNsYXNzPVwicGxheS1jb250YWluZXJcIj5cbiAgICA8ZWwtYnV0dG9uIEBjbGljaz1cImRyYXdlciA9IHRydWVcIj5PcGVuIERyYXdlcjwvZWwtYnV0dG9uPlxuICAgIDxlbC1kcmF3ZXIgdi1tb2RlbD1cImRyYXdlclwiIGFwcGVuZC10by1ib2R5PlxuICAgICAgPGVsLWJ1dHRvbiBAY2xpY2s9XCJkaWFsb2cgPSB0cnVlXCI+T3BlbiBEaWFsb2c8L2VsLWJ1dHRvbj5cbiAgICAgIDxlbC1kaWFsb2cgdi1tb2RlbD1cImRpYWxvZ1wiIGFwcGVuZC10by1ib2R5PkRpYWxvZzwvZWwtZGlhbG9nPlxuICAgIDwvZWwtZHJhd2VyPlxuICA8L2Rpdj5cbjwvdGVtcGxhdGU+XG5cbjxzY3JpcHQgbGFuZz1cInRzXCIgc2V0dXA+XG5pbXBvcnQgeyByZWYgfSBmcm9tICd2dWUnXG5cbmNvbnN0IGRyYXdlciA9IHJlZihmYWxzZSlcbmNvbnN0IGRpYWxvZyA9IHJlZihmYWxzZSlcbjwvc2NyaXB0PlxuIiwiaW1wb3J0LW1hcC5qc29uIjoie1wiaW1wb3J0c1wiOnt9fSJ9

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[github-actions]

[jw-foss]

Thank you for fixing this, I have been sloppy migrating this component, sorry 🙇