Merge #17049: contrib: Bump gitian descriptors for 0.20

fa1ad8f06e build: Bump gitian descriptor versions (MarcoFalke)

Pull request description:

  Bump the gitian descriptor versions as a follow-up to #17007.

  Also fixes #17027 with a cherry-pick, and bump the manpages.

ACKs for top commit:
  fanquake:
    ACK fa1ad8f06e

Tree-SHA512: c3b669c3797e5febb51a8dd01e2621a7544a291e080d73c47a2a12ea9da84ff904533e68792e2e869ebbdc2226b2fee7517214549e6cc7e988f175098f7c412c

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: