ci: Make compute resources for CI configurable

See prior commit for an explanation for the goal of the change and why it had
to be split into two commits.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/20230808021541.7lbzdefvma7qmn3w@awork3.anarazel.de
Backpatch: 15-, where CI support was added

.cirrus.tasks.yml

@@ -5,12 +5,6 @@
 
 
 env:
-  # Source of images / containers
-  GCP_PROJECT: pg-ci-images
-  IMAGE_PROJECT: $GCP_PROJECT
-  CONTAINER_REPO: us-docker.pkg.dev/${GCP_PROJECT}/ci
-  DISK_SIZE: 25
-
   # The lower depth accelerates git clone. Use a bit of depth so that
   # concurrent tasks and retrying older jobs have a chance of working.
   CIRRUS_CLONE_DEPTH: 500
@@ -29,45 +23,6 @@ env:
   PG_TEST_EXTRA: kerberos ldap ssl load_balance
 
 
-# Define how to run various types of tasks.
-
-# VMs provided by cirrus-ci. Each user has a limited number of "free" credits
-# for testing.
-cirrus_community_vm_template: &cirrus_community_vm_template
-  compute_engine_instance:
-    image_project: $IMAGE_PROJECT
-    image: family/$IMAGE_FAMILY
-    platform: $PLATFORM
-    cpu: $CPUS
-    disk: $DISK_SIZE
-
-
-default_linux_task_template: &linux_task_template
-  env:
-    PLATFORM: linux
-  <<: *cirrus_community_vm_template
-
-
-default_freebsd_task_template: &freebsd_task_template
-  env:
-    PLATFORM: freebsd
-  <<: *cirrus_community_vm_template
-
-
-default_windows_task_template: &windows_task_template
-  env:
-    PLATFORM: windows
-  <<: *cirrus_community_vm_template
-
-
-# macos workers provided by cirrus-ci
-default_macos_task_template: &macos_task_template
-  env:
-    PLATFORM: macos
-  macos_instance:
-    image: $IMAGE
-
-
 # What files to preserve in case tests fail
 on_failure_ac: &on_failure_ac
   log_artifacts: