start --> flex-start

Fixes postcss runtime warnings

app/styles/soapbox/about.scss

@@ -765,7 +765,7 @@ $small-breakpoint: 960px;
 
   &__brand {
     display: flex;
-    justify-content: start;
+    justify-content: flex-start;
     align-items: center;
     flex-direction: column;
 

app/styles/soapbox/basics.scss

@@ -142,7 +142,7 @@ button {
     display: flex;
     width: 100%;
     height: 100%;
-    align-items: start;
+    align-items: flex-start;
     justify-content: center;
     outline: 0 !important;
   }

app/styles/soapbox/components/user-panel.scss

@@ -77,7 +77,7 @@
 
   .user-panel-stats-item {
     display: flex;
-    align-items: start;
+    align-items: flex-start;
     justify-content: left;
     flex-wrap: wrap;
     flex-direction: column;