snap: fix license identifier

as of now, the snap ecosystem only supports SPDX 2.1 identifiers.

Change-Id: Iaa35b7d47d20bd320c585a2b72fa715b9bac2a5f

packaging/rules/snap/jami-gnome/snapcraft.yaml

@@ -48,7 +48,7 @@ name: jami-gnome
 version: "RELEASE_VERSION"
 adopt-info: jami
 icon: ../common/icons/jami.svg
-license: GPL-3.0-or-later
+license: GPL-3.0+
 summary: 'Jami: secure, distributed communication software & SIP client'
 description: >
   Jami is free software which allows its users to communicate in multiple ways.

packaging/rules/snap/jami/snapcraft.yaml

@@ -48,7 +48,7 @@ name: jami
 version: "RELEASE_VERSION"
 adopt-info: jami
 icon: ../common/icons/jami.svg
-license: GPL-3.0-or-later
+license: GPL-3.0+
 summary: 'Jami: secure, distributed communication software & SIP client'
 description: >
   Jami is free software which allows its users to communicate in multiple ways.