Measure CF-D upgrades appropriately with uptimer

[ctlong]

CF-D pipelines currently use uptimer to capture certain measurements relating to downtime during CF-D upgrades, and to fail them when those measurements exceed certain thresholds. However, due the retry logic we've added to deploys, the thresholds are not being applied appropriately – uptimer may fail a successful deploy, then on a retry the deploy will essentially be a no-op, resulting in no downtime for uptimer to fail on because no change occurred.

### Tasks
- [ ] Reinvestigate and update the thresholds we set on upgrades.
- [x] Remove retry logic from (upgrade?) deploys to ensure that uptimer works well.
- [x] Set reasonable value for `APP_STATS_THRESHOLD` in uptimer config
- [ ] Update uptimer errors post-tear down to improve clarity

[jochenehret]

Good point. I think we should remove the "retry" for the "bosh-deploy-cf-develop" tasks in the "upgrade-deploy" and "experimental-deploy" jobs.

The first attempts of "bosh-deploy-cf-develop" have indeed been failing often recently, mostly because of the newly introduced "Stats availability" test: cloudfoundry/uptimer#115.

[UPTIMER] 2024/05/10 02:16:16 FAILED (Stats availability): 2 failed attempts to retrieve stats for app exceeded the threshold of 0 allowed failures (Total attempts: 103, pass rate 98.06%)

The current default value for APP_STATS_THRESHOLD is set to 0: https://github.com/cloudfoundry/cf-deployment-concourse-tasks/blob/bbca3545a2a9781f6aa8635b9440a664f184a848/bosh-deploy/task.yml#L113

We should increase this value slightly. Not sure however what a reasonable threshold might be.

[jochenehret]

Opened PR: #1177

[ctlong]

@jochenehret called out that uptimer emits failure messages pretty consistently during the tear down, e.g. [UPTIMER] 2024/05/08 20:09:08 FAILURE (App pushability, 2/10): exit status 1, which are confusing coming right after the measurement summaries.

We should improve those messages, either

• Don't emit them, if possible.
• Make the message clear that the failure was during tear down only.

I've added a task to the issue description for this.