doc: ansible version dependency for stable-7.0 is changed to 2.15

The ansible version dependency in the stable-7.0 branch has been
upgraded to 2.15, and the ansible version dependency in the main branch
has also been upgraded to >=2.15,so update the documentation accordingly.

Close issue: #7594

Signed-off-by: bugwz <xbugwz@gmail.com>

docs/source/index.rst

@@ -93,9 +93,9 @@ The ``master`` branch should be considered experimental and used with caution.
 
 - ``stable-6.0`` Supports Ceph version ``pacific``. This branch requires Ansible version ``2.9``.
 
-- ``stable-7.0`` Supports Ceph version ``quincy``. This branch requires Ansible version ``2.12``.
+- ``stable-7.0`` Supports Ceph version ``quincy``. This branch requires Ansible version ``2.15``.
 
-- ``main`` Supports the main (devel) branch of Ceph. This branch requires Ansible version ``2.12``.
+- ``main`` Supports the main (devel) branch of Ceph. This branch requires Ansible version ``2.15`` or ``2.16``.
 
 .. NOTE:: ``stable-3.0`` and ``stable-3.1`` branches of ceph-ansible are deprecated and no longer maintained.