Don't enable fly mode on login when player is in water

shouldFly() treats water as unsafe blocks, which incorrectly
triggered fly mode for players logging in while swimming. Guard
the isInWater() call behind a version check since the method was
added in 1.17.

Fixes #6198

Author: JRoy

Essentials/src/main/java/com/earth2me/essentials/EssentialsPlayerListener.java

@@ -505,7 +505,8 @@ private void joinFlow(final User user, final long currentTime, final String mess
 
         if (user.isAuthorized("essentials.fly.safelogin")) {
             user.getBase().setFallDistance(0);
-            if (LocationUtil.shouldFly(ess, user.getLocation())) {
+            final boolean inWater = VersionUtil.getServerBukkitVersion().isHigherThanOrEqualTo(VersionUtil.v1_17_R01) && user.getBase().isInWater();
+            if (!inWater && LocationUtil.shouldFly(ess, user.getLocation())) {
                 user.getBase().setAllowFlight(true);
                 user.getBase().setFlying(true);
                 if (ess.getSettings().isSendFlyEnableOnJoin()) {