build/pkgs/babel/install-requires.txt: increase lower bound

Require at least babel-2.11.0 for the bugfix,

  * Use email.Message for pofile header parsing instead of the deprecated
    cgi.parse_header function. :gh:`876` - Aarni Koskela

This avoids deprecation warnings that cause some of our doctests to
fail; for example,

  sage: sage_getdoc(foo)
  DeprecationWarning: 'cgi' is deprecated and slated for removal in Python
  3.13
  'Construct a Foo.\n'

in src/doc/en/developer/coding_basics.rst.

build/pkgs/babel/install-requires.txt

@@ -1 +1 @@
-babel >=2.6.0
+babel >=2.11.0