v0.10.0-beta.5: Add support for Ping-Body-Limit response header

After the discussion in #32, Healthchecks added a response header that
sends the instances configured ping body limit.

Ping Body Limit
===============
- If the instance sent the header and user did not pass a limit via
  command-line flags, value in the header is used.

  /!\ This value is currently capped to 10MB by runitor.

      Underlying ring buffer implementation used for tailing the output
      eagerly allocates memory. If not capped, it can become a DoS
      vector from untrusted instances or in resource constrained runtime
      environments (VMs, containers, embedded systems, etc)

- If the user passed a limit and the instance sent its limit with the
  header, the lower of the two is used.

- If the instance doesn't send this header, and user doesn't specify an
  explicit value, we still default to 10KB.

Runitor discovers this limit with from the response of start ping. If
you're not sending start pings, then currently you can't use this
instance configuration discovery feature.

Go 1.18
=======
Due to use of generics, now the minimum Go version is 1.18

Tests
=====
Fixes a flaky and racy test