Use cloud benchmark machines for release report

[boshek]

This PR modifies the performance release report to report on benchmarks collected by cloud machines only. It removes both the ursa-i9-9960x and ursa-thinkcentre-m75q machines in favour of the ec2-m5-4xlarge-us-east-2 and ec2-c6a-4xlarge-us-east-2.

Because the previous release was not benchmarked on those machines we have initiated manual runs of those to facilitate the comparison.

Here are these two runs:

• https://conbench.ursa.dev/runs/austin-testin/
• https://conbench.ursa.dev/runs/austin-testin-c6a/

These are benchmarking apache/arrow@7dd1d34 as a baseline so any subsequent render of the release report should set
BASELINE_GIT_COMMIT=7dd1d34074af176d9e861a360e135ae57b21cf96

Then the CONTENDER_GIT_COMMIT can simply be the rc commit.

cc @raulcd and @assignUser

[raulcd]

Changes LGTM, not an expert on R nor quarto but I am going to merge as @assignUser might be out for some days and we will test this for 17.0.0. @boshek I'll ping you if something fails and we require a fix for the release.