[FIX] website_blog, *: allow more than 6 blog posts in dynamic snippet

*: website

Commit [1] introduced the possibility to configure "website filters"
used in the website to display some records dynamically via the "dynamic
snippet" and its variants. One of their field is `limit` which is a max
number of records the filter allows to show (so that the client side
cannot ask for thousands of records). Users can configure the limit used
by a snippet via the editor: they are allowed to choose up to 16, still
safe-guarded by the internal limit configured on the website filter on
the python side.

The problem here was that [2] introduced website filters for the blog
posts but set up a max limit of 6. Thus breaking the editor option if
the user choose a limit between 7 and 16. This commit fixes the issue,
for newly configured snippets (as a stable fix) or for users who would
-u their blog application.

Steps to reproduce:
- Install blog application
- Add a "Blog Posts" snippet
- Set "Fetched Elements" to 10 (there are 7 records in demo data)
=> Only 6 are still shown
- Save

With the fix:
- Restart your server
=> Only 6 are still shown
- Enter edit mode, reset "Fetched Elements" to >6
=> 7 records are now shown

[1]: https://github.com/odoo/odoo/commit/0e7640b5f22d2bea04bbe22d3189cff7e03af545
[2]: https://github.com/odoo/odoo/commit/3c0d98bcd8adf9325ee3497eb8d25ec7f904d6a5



opw-2885948

closes odoo/odoo#106754

Signed-off-by: Romain Derie (rde) <rde@odoo.com>