You wrote:
But , doxygen not accepted @startmindmap.
Doxygen has, as of version 1.9.2, the possibility to specify a so called engine (in this case mindmap) so one can write:
@startuml{mindmap} though you probably would like to keep the @startmindmap in your code and for this doxygen has the ALIASES settings possibility (to be set in your doxygen configuration file Doxyfile) like:
ALIASES += "startmindmap=@startuml{mindmap}"
See in the doxygen documentation:
https://www.doxygen.nl/manual/commands.html#cmdstartuml
https://www.doxygen.nl/manual/config.html#cfg_aliases