Rename <required> tag to <plugin>.

PiperOrigin-RevId: 509237786
Change-Id: I35e812830b479004ac5025cbd9477888f51fb43e

model/plugin/belt.xml

@@ -17,7 +17,7 @@
   <include file="scene.xml"/>
 
   <extension>
-    <required plugin="mujoco.elasticity.cable"/>
+    <plugin plugin="mujoco.elasticity.cable"/>
   </extension>
 
   <statistic center="0 0 .2" extent="1"/>
@@ -74,4 +74,4 @@
   <actuator>
     <motor site="S_last" gear="0 0 0 1 0 0" ctrlrange="-.2 .2"/>
   </actuator>
-</mujoco>
+</mujoco>

model/plugin/cable.xml

@@ -17,7 +17,7 @@
   <include file="scene.xml"/>
 
   <extension>
-    <required plugin="mujoco.elasticity.cable"/>
+    <plugin plugin="mujoco.elasticity.cable"/>
   </extension>
 
   <statistic center="0 0 .3" extent="1"/>

model/plugin/coil.xml

@@ -17,7 +17,7 @@
   <include file="scene.xml"/>
 
   <extension>
-    <required plugin="mujoco.elasticity.cable"/>
+    <plugin plugin="mujoco.elasticity.cable"/>
   </extension>
 
   <statistic center="0 0 .2" extent="1"/>
@@ -103,4 +103,4 @@
   <actuator>
     <position joint="slide" name="extend" gear="1 0 0 0 0 0" ctrlrange="-.25 1" kp="100"/>
   </actuator>
-</mujoco>
+</mujoco>

model/plugin/floppy.xml

@@ -17,7 +17,7 @@
   <include file="scene.xml"/>
 
   <extension>
-    <required plugin="mujoco.elasticity.solid"/>
+    <plugin plugin="mujoco.elasticity.solid"/>
   </extension>
 
   <compiler autolimits="true"/>

model/plugin/jelly.xml

@@ -17,7 +17,7 @@
   <include file="scene.xml"/>
 
   <extension>
-    <required plugin="mujoco.elasticity.solid"/>
+    <plugin plugin="mujoco.elasticity.solid"/>
   </extension>
 
   <option solver="CG" tolerance="1e-6" timestep=".001"/>

model/plugin/press.xml

@@ -17,7 +17,7 @@
   <include file="scene.xml"/>
 
   <extension>
-    <required plugin="mujoco.elasticity.solid"/>
+    <plugin plugin="mujoco.elasticity.solid"/>
   </extension>
 
   <compiler autolimits="true"/>