Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39504

We are receiving "killed" signals in CircleCI for the analyze_pr jobs.
Looking at the doc, it seems that it is due to us allocating to much memory.

As a mitigation, let's bumpt the executor to Medium.

## Changelog:
[Internal] - Bump executors for Analyze-PR to Medium to avoid choking

## Facebook:

Small machines consume 5 cred/min. Medium consume 10.
On average, the job takes 7 min to complete. So we are paying 35 credits per job. We will be paying 70 creds.
The upper bound cost is $ 0.042 per execution.
In the past month we were running 1820 jobs. We will be spending 1820 * 0.042  = $76.44 per month, rather than $38.22.
This is roughly $500/year.

We can fix the script with a different approach in the future.

Reviewed By: cortinico, dmytrorykun

Differential Revision: D49368118

fbshipit-source-id: 7e04e4be79b0f07d02338834e543b707c93c4363