mailmap: change primary address for Derrick Stolee

mailmap: change primary address for Derrick Stolee

Stolee transitioned from Microsoft to GitHub in July 2020, but continued
to use <dstolee@microsoft.com> because it was a valid address. He also
used <stolee@gmail.com> to communicate with the mailing list since
writing plaintext emails is difficult in Outlook. However, recent issues
with GMail delaying mailing list messages created a need to change his
primary email address.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>