Switch base OS to Debian bullseye amd64

* Also drop python-yaml (and python)… package doesn't exist anymore and
  we probably don't need system python2?
* gawk 5 warns when escaping `~`… and it wasn't needed on older gawk
  versions

Depends-On: 15547
Change-Id: Ib8a25d112ba1bfd75b8a5d005f2cfd93323e9e9f

.ci/Jenkinsfile

@@ -35,7 +35,7 @@ pipeline {
 		stage('Container Build') {
 			agent { label 'conviz1||conviz2' }
 			environment {
-				DOCKER_BASE_IMAGE = "debian:buster"
+				DOCKER_BASE_IMAGE = "debian:bullseye"
 				// FIXME: ^${DEPENDENCY_PYTHON} is a workaround for an invalid spectrum-mpi concretization
 				DEPENDENCY_PYTHON = "python@2.7.18"
 				// This needs to be here because otherwise the default python

.ci/commons.sh

@@ -639,7 +639,7 @@ get_latest_hash() {
   # insert compiler name into spec name at appropriate position (i.e., prior to
   # specifying any variants)
   # \$1 is the hash, \$2 is the spec
-  idx = match(\$2, /(\\+|\\~|$)/);
+  idx = match(\$2, /(\\+|~|$)/);
   printf("%s%%%s%s /%s\\n", substr(\$2, 0, idx-1), compiler, substr(\$2, idx), \$1)
 }
 EOF

.ci/install_prerequisites.sh

@@ -25,8 +25,6 @@ prerequisites=(
     "patch"
     "patchelf"
     "procps"
-    "python"
-    "python-yaml"
     "python3"
     "python3-yaml"
     "rsync"