BYO certs - Changed to support v3 certs with SAN

[benc-uk]

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Description

Since Go v1.15.2 it is expected that certs have a SAN field (see this issue), the instructions in the current mTLS docs for bringing your own cert do not generate certs in this format or with this field. This results in errors and warnings when Sentry starts and also in the injected sidecars

This PR provides instructions and commands to generate v3 certs with the correct SAN extension field

The instructions now use the openssl command rather than step as it's not easy to generate v3 certs with step.

Issue reference

Fixes #1498