maint: don’t use \] in BREs and EREs

* .travis.yml (jobs):
* doc/local.mk ($(top_srcdir)/%D%/bison.help):
* tests/regression.at (Web2c Actions, Useless Tokens):
* tests/scanner.at (Token numbers: $1):
Use plain ], not \], in a POSIX regular expression,
as POSIX says the interpretation of \] is undefined.

.travis.yml

@@ -45,7 +45,7 @@ jobs:
         - git submodule update --init --recursive
 
         # As of 2021-08, we don't have Autoconf 2.71 on Ubuntu.
-        - sed -i 's/AC_PREREQ(\[2\.71\])/AC_PREREQ([2.69])/g' configure.ac
+        - sed -i 's/AC_PREREQ(\[2\.71])/AC_PREREQ([2.69])/g' configure.ac
         - ./bootstrap
         - ./configure --enable-gcc-warnings || { cat config.log && false; }
         - make -j2