MDL-69479 core: Change 4.0 to 3.10 in theme/upgrade.txt

theme/upgrade.txt

 This files describes API changes in /theme/* themes,
 information provided here is intended especially for theme designer.
 
-=== 4.0 ===
+=== 3.10 ===
 * The Bootstrap legacy css utilities from Bootstrap 2 and 4alpha have been removed.
 The syntax for the new Bootstrap 4.5 utility classes is {property}{sides}-{breakpoint}-{size} for sm, md, lg, and xl.
 The size values are: