fix(docs): deprecate plat_convert_pk() in v2.9

The deprecation was tagged "Next release after 2.8". Now there is a 2.9
planned, directly use this version.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I0727eebc4a3800dafafc4166b0c2c40a12c90b4b