make_obs_url: fix platform for newer Leap releases boo#1219148

[lkocman]

Result of https://lists.opensuse.org/archives/list/project@lists.opensuse.org/thread/RIUQNUEQ2SQ2UT4Q3RIJVHISUY3IAJGV/
Tracked as https://bugzilla.suse.com/show_bug.cgi?id=1219148

We no longer use openSUSE_Leap_15.6 for Leap repositories. Nowadays it's just 15.6 (in fact since 15.3 I believe).

lkocman@localhost:~> sudo zypper ar obs://devel:tools:ide:vscode devel_tools_ide_vscode
[sudo] password for root: 
Guessed: platform = openSUSE_Leap_$releasever
Adding repository 'devel_tools_ide_vscode' ................................................................................................................................................................[done]
Repository 'devel_tools_ide_vscode' successfully added

URI         : https://download.opensuse.org/repositories/devel:/tools:/ide:/vscode/openSUSE_Leap_15.6
Enabled     : Yes
GPG Check   : Yes
Autorefresh : No
Priority    : 99 (default priority)

Repository priorities are without effect. All enabled repositories share the same priority.

Result returns 404 https://download.opensuse.org/repositories/devel:/tools:/ide:/vscode/openSUSE_Leap_15.6

as the url should be https://download.opensuse.org/repositories/devel:/tools:/ide:/vscode/15.6

[lkocman]

I suppose we do not care about older releases which are EOL nowadays. 15.4 was marked as EOL on Dec 31 2023.