doc: update buildkite instance type (#4459)

Documenting that we've bumped the buildkite instance type from t3 to
m6i.
The main reason for the switch is that the "burstable" nature of the
t*-instances makes them extremely unsuitable for running benchmarks. The
m6i are also much faster, but not much more expensive.
Also, the agent pool is now at most 6 agents. Spinning up 10 agents and
having to warm up all the caches, and then scaling back in after a
single build, seemed excessive.

doc/dev/testing/buildkite.rst

@@ -89,7 +89,8 @@ Cluster configuration
 
    The agent cluster is operated by the SCION Association, in the AWS account ``scion-association``.
 
-   Primary contact `matzf <https://github.com/matzf>`_, alt contact `nicorusti <https://github.com/nicorusti>`_.
+   Primary contacts `matzf <https://github.com/matzf>`_, `jiceatscion <https://github.com/jiceatscion>`_,
+   alt contact `nicorusti <https://github.com/nicorusti>`_.
 
 
 The agent cluster is based on the `buildkite AWS CloudFormation template <https://buildkite.com/docs/agent/v3/elastic-ci-aws/parameters>`_.
@@ -99,15 +100,15 @@ Excerpt of the most relevant parameters:
 
    # Instance Configuration:
    ImageId: ""               # use default machine image based on Amazon Linux 2
-   InstanceType: t3.2xlarge  # 8 vCPUs, 32GiB memory
+   InstanceType: m6i.2xlarge # 8 vCPUs, 32GiB memory
    RootVolumeSize: 100
    RootVolumeType: gp3
    BuildkiteAdditionalSudoPermissions: ALL  # allow any sudo commands in pipeline
    EnableDockerUserNamespaceRemap: false    # not compatible with using host network namespace
 
    # Auto-scaling Configuration:
    MinSize: 0
-   MaxSize: 10
+   MaxSize: 6
    ScaleInIdlePeriod: 600  # shut down after 10 minutes idle
-   ScaleOutFactor: 0.5
+   ScaleOutFactor: 1.0
    OnDemandPercentage: 0   # use only spot instances