build: Bump gitian descriptor versions

contrib/gitian-descriptors/gitian-linux.yml

@@ -1,5 +1,5 @@
 ---
-name: "bitcoin-core-linux-0.19"
+name: "bitcoin-core-linux-0.20"
 enable_cache: true
 distro: "ubuntu"
 suites:

contrib/gitian-descriptors/gitian-osx.yml

@@ -1,5 +1,5 @@
 ---
-name: "bitcoin-core-osx-0.19"
+name: "bitcoin-core-osx-0.20"
 enable_cache: true
 distro: "ubuntu"
 suites:

contrib/gitian-descriptors/gitian-win.yml

@@ -1,5 +1,5 @@
 ---
-name: "bitcoin-core-win-0.19"
+name: "bitcoin-core-win-0.20"
 enable_cache: true
 distro: "ubuntu"
 suites: