Add Mikko as a maintainer

[julianocosta89]

I'd like to move @mviitane from approver to maintainer.

Mikko has been involved in the OTel Demo since the beginning of it, has been actively contributing to the project and an active approver.

He would replace @cartersocha and we'd move Carter to emeritus maintainer status.

[austinlparker]

👍 sgtm

[puckpuck]

I agree to doing this