[linter] Bump prettier minimum version to 2.6.0

Summary: There are differences in the output against the 2.4.1 version which is currently enforced, so let's bump to the new minor. See D13834.

Test Plan:

arc lint -- web/explorer/explorer-server/templates/pages/address.html

Should return no change.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D13837