libdecnumber: replace FSF postal address with licenses URL

Some of the GPLv2 boiler-plate still contained the
obsolete "51 Franklin Street" postal address.

Replace it with the canonical GNU licenses URL recommended by the FSF:
https://www.gnu.org/licenses/

Signed-off-by: Sean Wei <me@sean.taipei>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250613.qemu.patch.03@sean.taipei>
Signed-off-by: Thomas Huth <thuth@redhat.com>

libdecnumber/decContext.c

@@ -24,9 +24,8 @@
    for more details.
 
    You should have received a copy of the GNU General Public License
-   along with GCC; see the file COPYING.  If not, write to the Free
-   Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
-   02110-1301, USA.  */
+   along with GCC; see the file COPYING.  If not, see
+   <https://www.gnu.org/licenses/>.  */
 
 /* ------------------------------------------------------------------ */
 /* Decimal Context module					      */

libdecnumber/decNumber.c

@@ -24,9 +24,8 @@
    for more details.
 
    You should have received a copy of the GNU General Public License
-   along with GCC; see the file COPYING.  If not, write to the Free
-   Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
-   02110-1301, USA.  */
+   along with GCC; see the file COPYING.  If not, see
+   <https://www.gnu.org/licenses/>.  */
 
 /* ------------------------------------------------------------------ */
 /* Decimal Number arithmetic module				      */

libdecnumber/dpd/decimal128.c

@@ -24,9 +24,8 @@
    for more details.
 
    You should have received a copy of the GNU General Public License
-   along with GCC; see the file COPYING.  If not, write to the Free
-   Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
-   02110-1301, USA.  */
+   along with GCC; see the file COPYING.  If not, see
+   <https://www.gnu.org/licenses/>.  */
 
 /* ------------------------------------------------------------------ */
 /* Decimal 128-bit format module				      */

libdecnumber/dpd/decimal32.c

@@ -24,9 +24,8 @@
    for more details.
 
    You should have received a copy of the GNU General Public License
-   along with GCC; see the file COPYING.  If not, write to the Free
-   Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
-   02110-1301, USA.  */
+   along with GCC; see the file COPYING.  If not, see
+   <https://www.gnu.org/licenses/>.  */
 
 /* ------------------------------------------------------------------ */
 /* Decimal 32-bit format module					      */

libdecnumber/dpd/decimal64.c

@@ -24,9 +24,8 @@
    for more details.
 
    You should have received a copy of the GNU General Public License
-   along with GCC; see the file COPYING.  If not, write to the Free
-   Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
-   02110-1301, USA.  */
+   along with GCC; see the file COPYING.  If not, see
+   <https://www.gnu.org/licenses/>.  */
 
 /* ------------------------------------------------------------------ */
 /* Decimal 64-bit format module					      */