Create a 13.5-specific update path

See tor-browser#44668 (comment 3352381).

We need to ensure that 13.5 users that have not yet reached the final 13.5 release still receive an upgrade path to the latest release. I.e. these users require an <update> entry for the final 13.5 version in the update-*.xml response.

We will probably need to add a watershed-like RewriteRule, except that we'll have to redirect 13.5.29 users to the usual updates (starting with 15.0.8).

/cc @boklm @pierov feel free to change the title and description, since I'm not sure about the correct wording or what work needs to be done.