Move 2d and 3d plugin force computation to elasticity.h.

Change physical properties of `floppy_flex.xml` to resemble `floppy.xml`.

Make `Membrane` backward compatible with `Composite`.

PiperOrigin-RevId: 577190384
Change-Id: I5bc56ec62faa134156e839bec45d685c22eba5c4

model/plugin/elasticity/floppy_flex.xml

@@ -22,7 +22,7 @@
 
   <compiler autolimits="true"/>
 
-  <option solver="Newton" tolerance="1e-6" timestep=".001" integrator="Euler"/>
+  <option solver="CG" tolerance="1e-6" timestep=".001"/>
 
   <size memory="100M"/>
 
@@ -32,7 +32,7 @@
 
   <worldbody>
     <flexcomp type="grid" count="24 4 4" spacing=".1 .1 .1" pos=".1 0 1.5"
-              radius=".0" rgba="0 .7 .7 1" name="softbody" dim="3" mass="7">
+              radius=".0" rgba="0 .7 .7 1" name="softbody" dim="3" mass="25">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none"/>
       <plugin plugin="mujoco.elasticity.solid">
         <config key="poisson" value="0.2"/>