[lao_phonetic] build with 15.0 if we want to target web [low-pri]

[mcdurdin]

This is low priority, just noting a potential issue for the future.

Due to keymanapp/keyman#5440, the lao_phonetic keyboard would not work correctly if it was built for web.

This is a bug in the 14.0 and earlier version compilers, which has been fixed in 15.0 (see #5525). Once we have a 15.0 compiler in the keyboards repository, we can rebuild this keyboard and make it available for web. (I note the keyboard is also not currently available for linux, which is a shame.)

I have audited all the keyboards in the repository, and this is the only one that is impacted by the described bug.