Bump ansible-core to 2.12.8

On 30.09.2023, "old" ansible galaxy(galaxy.ansible.com) was replaced
with "galaxy ng"[1].
There was a bug [2][3][4] that was fixed only in a supported
ansible-core versions, so:
- v2.12.8
- v2.13.3
- v2.14.0 (and all newer versions)

All openstack-ansible versions up to Yoga are using older ansible-core
versions.
As a result, bootstrap-ansible.sh script fails with an error:

```
Skipping Galaxy server https://galaxy.ansible.com/api/. Got an
unexpected error when getting available versions of collection
ansible.netcommon:
'/api/v3/plugin/ansible/content/published/collections/index/ansible/netcommon/versions/'
```

This patch bumps ansible-core version to 2.12.8 that includes required
fix.

[1] https://www.ansible.com/blog/new-ansible-galaxy
[2] https://github.com/ansible/ansible/issues/81830
[3] https://github.com/ansible/ansible/issues/77911
[4] https://github.com/ansible/ansible/pull/78325

Closes-Bug: #2038315
Change-Id: Ic381b98f92c12899da4cd64873989522151cb7eb

scripts/bootstrap-ansible.sh

@@ -22,7 +22,7 @@ set -e -u -x
 export HTTP_PROXY=${HTTP_PROXY:-""}
 export HTTPS_PROXY=${HTTPS_PROXY:-""}
 # The Ansible version used for testing
-export ANSIBLE_PACKAGE=${ANSIBLE_PACKAGE:-"ansible-core==2.12.6"}
+export ANSIBLE_PACKAGE=${ANSIBLE_PACKAGE:-"ansible-core==2.12.8"}
 export ANSIBLE_ROLE_FILE=${ANSIBLE_ROLE_FILE:-"ansible-role-requirements.yml"}
 export ANSIBLE_COLLECTION_FILE=${ANSIBLE_COLLECTION_FILE:-"ansible-collection-requirements.yml"}
 export USER_ROLE_FILE=${USER_ROLE_FILE:-"user-role-requirements.yml"}