Update error and usage alarms

[molly-moen]

This PR makes the following updates:

• Remove the high usage alarms. These did not provide a useful signal and high usage is covered by the high concurrent executions alarm
• Update the high concurrent execution alarm to a threshold of 400 concurrent executions for 10 minutes for a single build and run lambda. Previously it alarmed if there were 50 concurrent executions across all lambdas, which was too low. The alarm is evaluated so that if we see 400 concurrent executions at least once each minute for 10 minutes, it will alarm and page the DOTD.
• Update the elevated severe error rate and elevated error rate alarms to page the DOTD instead of post to slack. These have been up and running for a week now with no issues.
• include a link to the debug steps for DOTD in the error rate and high concurrent execution alarms

Testing

Tested on a dev instance