deprecate spot::acc_cond::format()

* NEWS: Mention it.
* spot/twa/acc.hh (spot::acc_cond::format): Deprecate.
(spot::acc_cond::mark_t::as_string): New function.
* spot/taalgos/dot.cc: Use mark_t::as_string().
* spot/priv/satcommon.cc, spot/priv/satcommon.hh,
spot/twaalgos/dtwasat.cc, spot/twaalgos/emptiness.cc,
tests/core/acc.cc, tests/core/acc.test: Adjust to use << directly.

NEWS

@@ -140,7 +140,7 @@ New in spot 2.7.5.dev (not yet released)
     properties.  This can be altered with the SPOT_PR_CHECK
     environment variable.
 
-  Backward incompatibilities:
+  Deprecation notices:
 
   - The virtual function twa::intersecting_run() no longuer takes a
     second "from_other" Boolean argument.  This is a backward
@@ -149,6 +149,11 @@ New in spot 2.7.5.dev (not yet released)
     call this function with two argument, a non-virtual version of the
     function has been introduced and marked as deprecated.
 
+  - The spot::acc_cond::format() methods have been deprecated.  These
+    were used to display acceptance marks, but acceptance marks are
+    unrelated to acceptance conditions, so it's better to simply print
+    marks with operator<< without this extra step.
+
   Bugs fixed:
 
   - The gf_guarantee_to_ba() is relying on an inplace algorithm that