Fix Athenian Long Walls technology following rP23843.
rP23843 changed modifiers to explicitly support only string or numeric values.
However, buildRestrictions territories worked with a list, which errored out. This is now replaced with a string.
Reported by: Asger
Investigated by: Freagarach.
Reviewed By: Freagarach
Fixes #5849
Differential Revision: https://code.wildfiregames.com/D3062