Add option to set minimum supported branch

[sbidoul]

At some point we will not want to continue generating READMEs and publishing wheels for older Odoo versions.

[legalsylvain]

Naive question : why ?

[sbidoul]

The short answer is that we can't support old versions forever.

In more details

• when I'll deploy the new README.rst generator, it will generate runboat links and runboat does not support Odoo 8 and 9 (it could but nobody contributed nor asked for it)
• the oca/oca-ci image does not support Odoo 8 and 9 (it could but nobody contributed nor asked for it)
• the OCA repo template does not support 8 and 9
• sooner or later we'll stop paying for travis so we won't have CI anymore for 8 and 9, without official alternative since no one tests the repo template with 8 and 9

So I'm going to ask that we declare 8 and 9 officially unsupported by OCA.

[sbidoul]

Ah, I forgot to mention that the main branch bot consumes a significant amount of resources on the OCA server, so it will help with that too.

Nevertheless this PR does not change the default behaviour, it just adds a configuration option.