Remove flex version checks

Remove the flex version checks from configure and meson.  The cutoff
versions are all so ancient that this is no longer relevant, and what
the actual cutoff should be is a bit fuzzy.

This also removes the ancient behavior that configure would also
accept a "lex" program if it is actuall flex.  This aligns the check
with meson in this respect.

For future reference, as of this commit, these are relevant flex
versions:

- The hard required minimum is flex 2.5.34 as of commit b1ef48980d,
  but this has not actually been tested.

- Prior to this, the minimum enforced by configure/meson was flex
  2.5.35, which is the oldest present in the buildfarm right now.

- As of commit 6fdd5d9563, the oldest version that will compile
  without warnings due to flex-generated code is flex 2.5.36.

- The oldest version that probably still has some practical relevance
  is flex 2.5.37, which ships with CentOS/RHEL 7.

Discussion: https://www.postgresql.org/message-id/1a204ccd-7ae6-478c-a431-407b5c48ccc6@eisentraut.org

doc/src/sgml/installation.sgml

@@ -100,10 +100,10 @@
        <primary>yacc</primary>
       </indexterm>
 
-      <application>Flex</application> 2.5.35 or later and
-      <application>Bison</application> 2.3 or later are required.  Other
-      <application>lex</application> and <application>yacc</application>
-      programs cannot be used.
+      <application>Flex</application> and <application>Bison</application> are
+      required.  Other <application>lex</application> and
+      <application>yacc</application> programs cannot be used.
+      <application>Bison</application> needs to be at least version 2.3.
      </para>
     </listitem>
 
@@ -3811,10 +3811,8 @@ make: *** [postgres] Error 1
         <productname>Flex</productname></term>
        <listitem>
        <para>
-        <productname>Bison</productname> and <productname>Flex</productname> are
-        required.  Only <productname>Bison</productname> versions 2.3 and later
-        will work. <productname>Flex</productname> must be version 2.5.35 or later.
-        Binaries can be downloaded from <ulink
+        Binaries for <productname>Bison</productname> and
+        <productname>Flex</productname> can be downloaded from <ulink
         url="https://github.com/lexxmark/winflexbison"></ulink>.
        </para>
        </listitem>