Remove f-string from DAP

One more f-string snuck into the DAP code, in breakpoint.py.  Most of
them were removed here:

    https://sourceware.org/pipermail/gdb-patches/2023-June/200023.html

but I think this one landed after that patch.

While DAP only supports Python 3.5 and later, f-strings were added in
3.6, so remove this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30708

gdb/python/lib/gdb/dap/breakpoint.py

@@ -352,7 +352,7 @@ def _catch_exception(filterId, **args):
     if filterId in ("assert", "exception", "throw", "rethrow", "catch"):
         cmd = "-catch-" + filterId
     else:
-        raise Exception(f"Invalid exception filterID: {filterId}")
+        raise Exception("Invalid exception filterID: " + str(filterId))
     result = gdb.execute_mi(cmd)
     # A little lame that there's no more direct way.
     for bp in gdb.breakpoints():