ci: windows: Use the same image for VS and MinGW tasks

The VS and MinGW Windows images have been merged, to reduce the space needed
for images. Before 98811323c8 the split helped boot performance, but now that
we are using VMs that doesn't appear to be the case anymore.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAN55FZ2kWYjPd7uUC5QswrB3tfVJDiURqC%2BMGM6a3oeev%3DVgOA%40mail.gmail.com
Backpatch: 15-, where CI was added

.cirrus.tasks.yml

@@ -520,6 +520,7 @@ WINDOWS_ENVIRONMENT_BASE: &WINDOWS_ENVIRONMENT_BASE
     PG_TEST_USE_UNIX_SOCKETS: 1
     PG_REGRESS_SOCK_DIR: "c:/cirrus/"
     DISK_SIZE: 50
+    IMAGE_FAMILY: pg-ci-windows-ci
 
   sysinfo_script: |
     chcp
@@ -543,7 +544,6 @@ task:
     # given that it explicitly prevents crash dumps from working...
     # 0x8001 is SEM_FAILCRITICALERRORS | SEM_NOOPENFILEERRORBOX
     CIRRUS_WINDOWS_ERROR_MODE: 0x8001
-    IMAGE_FAMILY: pg-ci-windows-ci-vs-2019
 
   <<: *windows_task_template
 
@@ -604,7 +604,6 @@ task:
     # Start bash in current working directory
     CHERE_INVOKING: 1
     BASH: C:\msys64\usr\bin\bash.exe -l
-    IMAGE_FAMILY: pg-ci-windows-ci-mingw64
 
   <<: *windows_task_template