verilog: Lower required bison version to 3.6

We're currently on version 3.6 of bison at Google, and Yosys
still correctly builds with it. This should better reflect
the actual requirements rather than an overly restrictive
check. If features from 3.8 are required it seems like bumping
would be appropriate.

Signed-off-by: Ethan Mahintorabi <ethanmoon@google.com>

frontends/verilog/verilog_parser.y

@@ -33,7 +33,7 @@
  *
  */
 
-%require "3.8"
+%require "3.6"
 %language "c++"
 %define api.value.type variant
 %define api.prefix {frontend_verilog_yy}