Merge #18586: build: Bump gitian descriptors to 0.21

dabe2bb11a build: Bump gitian descriptors to 0.21 (Wladimir J. van der Laan)

Pull request description:

  Per the release process.

ACKs for top commit:
  MarcoFalke:
    ACK dabe2bb11a, same as last bump: fa1ad8f06e

Tree-SHA512: 6548aa6f6adb31ac98e65290060c5a8a84e2c44e6634ddcc0299cea8f32be6966c7dba91696e1e7b759d251ccba5ac421c949ed0f62b7b6d07b7c2b3ef51248b

contrib/gitian-descriptors/gitian-linux.yml

@@ -1,5 +1,5 @@
 ---
-name: "bitcoin-core-linux-0.20"
+name: "bitcoin-core-linux-0.21"
 enable_cache: true
 distro: "ubuntu"
 suites:

contrib/gitian-descriptors/gitian-osx.yml

@@ -1,5 +1,5 @@
 ---
-name: "bitcoin-core-osx-0.20"
+name: "bitcoin-core-osx-0.21"
 enable_cache: true
 distro: "ubuntu"
 suites:

contrib/gitian-descriptors/gitian-win.yml

@@ -1,5 +1,5 @@
 ---
-name: "bitcoin-core-win-0.20"
+name: "bitcoin-core-win-0.21"
 enable_cache: true
 distro: "ubuntu"
 suites: