fix: ES5 compatibility

Author: MikeyParton

src/index.js

@@ -1017,7 +1017,7 @@ class MiniCssExtractPlugin {
                           [
                             `var href = ${RuntimeGlobals.require}.miniCssF(chunkId);`,
                             `var fullhref = ${RuntimeGlobals.publicPath} + href;`,
-                            'const oldTag = findStylesheet(href, fullhref);',
+                            'var oldTag = findStylesheet(href, fullhref);',
                             'if(!oldTag) return;',
                             `promises.push(new Promise(${runtimeTemplate.basicFunction(
                               'resolve, reject',

test/cases/hmr/expected/webpack-5/main.js

@@ -914,7 +914,7 @@ module.exports = function (urlString) {
 /******/ 			chunkIds.forEach((chunkId) => {
 /******/ 				var href = __webpack_require__.miniCssF(chunkId);
 /******/ 				var fullhref = __webpack_require__.p + href;
-/******/ 				const oldTag = findStylesheet(href, fullhref);
+/******/ 				var oldTag = findStylesheet(href, fullhref);
 /******/ 				if(!oldTag) return;
 /******/ 				promises.push(new Promise((resolve, reject) => {
 /******/ 					var tag = createStylesheet(chunkId, fullhref, () => {