Fixed syntax for before-after operators in less files.

Author: sanganinamrata

app/design/adminhtml/Magento/backend/web/app/setup/styles/less/lib/forms/_checkbox-radio.less

@@ -87,7 +87,7 @@
 .form-el-checkbox {
     &:checked {
         + .form-label {
-            &::before {
+            &:before {
                 content: @checkbox-icon__content;
                 font-family: @icons__font-family;
             }

app/design/adminhtml/Magento/backend/web/app/updater/styles/less/components/_data-grid.less

@@ -22,8 +22,8 @@
     vertical-align: middle;
     width: @component-indicator__size;
 
-    &::before,
-    &::after {
+    &:before,
+    &:after {
         background: @color-white;
         display: block;
         opacity: 0;
@@ -32,7 +32,7 @@
         visibility: hidden;
     }
 
-    &::before {
+    &:before {
         border: 1px solid @color-gray68;
         border-radius: 1px;
         box-shadow: 0 0 2px rgba(0,0,0,.4);
@@ -43,7 +43,7 @@
         padding: 4px 5px;
     }
 
-    &::after {
+    &:after {
         border-color: darken(@color-gray68, 8);
         border-style: solid;
         border-width: 1px 0 0 1px;
@@ -56,8 +56,8 @@
     }
 
     &:hover {
-        &::before,
-        &::after {
+        &:before,
+        &:after {
             opacity: 1;
             transition: opacity .2s linear;
             visibility: visible;

app/design/frontend/Magento/blank/Magento_Checkout/web/css/source/module/checkout/_authentication.less

@@ -94,7 +94,7 @@
                 padding-top: @indent__xl;
                 position: relative;
 
-                &::before {
+                &:before {
                     .lib-css(height, @block-auth__or-label__size);
                     .lib-css(line-height, @block-auth__or-label__size - 2px);
                     .lib-css(margin, -(@block-auth__or-label__size/2 + 1px) 0 0 -(@block-auth__or-label__size / 2));
@@ -212,7 +212,7 @@
                 margin: 0;
                 padding: @indent__s 0 0 @indent__xl;
 
-                &::before {
+                &:before {
                     left: 0;
                     top: 50%;
                 }