Standardize references to Zstandard as <productname>

Some places used ZSTD, which isn't widely used anywhere.  Use ZSTD only
to refer to the environment variable; use zstd (all lowercase) to refer
to the utility.

Per complaint from Justin Pryzby.

Discussion: https://postgr.es/m/20220414003301.GT26620@telsasoft.com

Author: alvherre

doc/src/sgml/install-windows.sgml

@@ -307,10 +307,10 @@ $ENV{MSBFLAGS}="/m";
     </varlistentry>
 
     <varlistentry>
-     <term><productname>ZSTD</productname></term>
+     <term><productname>Zstandard</productname></term>
      <listitem><para>
-      Required for supporting <productname>ZSTD</productname> compression
-      method. Binaries and source can be downloaded from
+      Required for supporting <productname>Zstandard</productname> compression.
+      Binaries and source can be downloaded from
       <ulink url="https://github.com/facebook/zstd/releases"></ulink>.
      </para></listitem>
     </varlistentry>

doc/src/sgml/installation.sgml

@@ -273,7 +273,7 @@ su - postgres
 
     <listitem>
      <para>
-      You need <productname>zstd</productname>, if you want to support
+      You need <productname>Zstandard</productname>, if you want to support
       compression of data with that method; see
       <xref linkend="guc-wal-compression"/>.
       The minimum required version is 1.4.0.
@@ -996,7 +996,7 @@ build-postgresql:
        <term><option>--with-zstd</option></term>
        <listitem>
         <para>
-         Build with <productname>ZSTD</productname> compression support.
+         Build with <productname>Zstandard</productname> compression support.
         </para>
        </listitem>
       </varlistentry>