Replays: Restore support for -restoreboost

I was watching an old replay and noticed it no longer worked.

Turn 7 of https://replay.pokemonshowdown.com/randombattle-247965078

Apparently when I did this rename, I didn't bother to make it
backwards compatible with old replays?

9aba6a3

src/battle-text-parser.ts

@@ -101,6 +101,11 @@ class BattleTextParser {
 		return {group, name, away, status};
 	}
 
+	/**
+	 * Old replays may use syntax we no longer use, so this function upgrades
+	 * them to modern versions. Used to keep battle.ts itself cleaner. Not
+	 * guaranteed to mutate or not mutate its inputs.
+	 */
 	static upgradeArgs({args, kwArgs}: {args: Args, kwArgs: KWArgs}): {args: Args, kwArgs: KWArgs} {
 		switch (args[0]) {
 		case '-activate': {
@@ -189,6 +194,11 @@ class BattleTextParser {
 			break;
 		}
 
+		case '-restoreboost': {
+			args[0] = '-clearnegativeboost';
+			break;
+		}
+
 		case '-nothing':
 			// OLD: |-nothing
 			// NEW: |-activate||move:Splash