[doc] Bump minimum Verilator version

It turns out that we use split_var in the code, which was added in
version 4.030 (and several bugs were fixed in following versions).
Change the minimum required version to match what we're using in
CI (and presumably works!)

This was triggered by issue #2080.

tool_requirements.py

@@ -5,7 +5,7 @@
 # Version requirements for various tools. Checked by tooling (e.g. fusesoc),
 # and inserted into the Sphinx-generated documentation.
 __TOOL_REQUIREMENTS__ = {
-    'verilator': '4.028',
+    'verilator': '4.104',
     'edalize':   '0.2.0',
     'vcs': {
         'min_version': '2020.03-SP2',