resigning as 13u/15u lead maintainer

[Yuri Nesterenko]

Hi,

according to the initial plans, now after January, 2023 release
I'm stepping down as lead maintainer of OpenJDK 13u and 15u.

As usual, if any suitably qualified individual wishes to take
over this role, they may apply to do so.

NB: (1) In the current state, pairs of master/team repositories 15u and 15u-dev,
13u and 13u-dev are similar to each other with identical tags.
To continue, one should request new release numbers.
     (2) If there will be no volunteers, we should archive 13u and 15u
repositories and probably delete 13u-dev and 15u-dev.

Thank you!

--yan