style(Field): label color updated to gray-7 (#6581)

src/field/index.less

@@ -9,6 +9,7 @@
     flex: none;
     box-sizing: border-box;
     width: @field-label-width;
+    color: @field-label-color;
     text-align: left;
 
     &--center {

src/style/var.less

@@ -346,6 +346,7 @@
 
 // Field
 @field-label-width: 90px;
+@field-label-color: @gray-7;
 @field-input-text-color: @text-color;
 @field-input-error-text-color: @red;
 @field-input-disabled-text-color: @gray-5;