[telega-bridge-bot] fix: get text without format

[BlindingDark]

Username may not be a plain string, for example

#("     " 0 2 (telega-display "𝐿") 2 4 (telega-display "𝑢") 4 6 (telega-display "𝑐") 6 8 (telega-display "𝑖") 8 10 (telega-display "𝑢") 10 12 (telega-display "𝑠"))

This can cause problems since we use it as a hash table key.
So here we use (telega-tl-str content-text :text 'no-props) to get the string without props.

[zevlg]

Merged in, thanks!