sd-bus: mark sd_bus_try_close() as deprecated

codesearch.debian.net shows no uses (except for the definition in systemd and
elogind).

$ cat > test.c

int main() {
  sd_bus_try_close(NULL);
  return 0;
}
$ gcc -Isrc/systemd -Wall -o testbus test.c -lsystemd
test.c: In function ‘main’:
test.c:4:3: warning: ‘sd_bus_try_close’ is deprecated [-Wdeprecated-declarations]
    4 |   sd_bus_try_close(NULL);
      |   ^~~~~~~~~~~~~~~~
In file included from test.c:1:
src/systemd/sd-bus.h:180:5: note: declared here
  180 | int sd_bus_try_close(sd_bus *bus) _sd_deprecated_; /* deprecated */
      |     ^~~~~~~~~~~~~~~~