Replace deprecated locate usage

[oluceps]

reference: typst/typst#6086

error: only element functions can be used as selectors
    ┌─ ../poster.typ:120:29
    │  
120 │     show heading: it => locate(loc => {
    │ ╭──────────────────────────────^
121 │ │     // Find out the final number of the heading counter.
122 │ │     let levels = counter(heading).at(loc)
123 │ │     let deepest = if levels != () {
    · │
160 │ │     ]
161 │ │   })
    │ ╰───^