Adjust apdex monitors after NR is turned off

[timmc-edx]

Now that the performance penalty of dual-instrumentation is gone, we can tighten up our apdex monitors again. (We had to loosen them when both DD and NR were enabled.)

Acceptance Criteria:

• Recreate sustained alerts for Arch-BOM in Datadog #830 should be completed first.
• Research the apdex monitors and see if we can tighten the thresholds
  • In particular, https://app.datadoghq.com/monitors/146372908 was adjusted from a threshold of 0.75 down to 0.50 due to noisiness in December 2024. Check what the more appropriate fix would be.
• If we can, experiment with lowering them and ensuring that the alerts don't get too noisy.

[robrap]

Note: Maybe we'll want to remind other teams about this as well?

[dianakhuang]

Blocked while we still have New Relic running

[robrap]

Blocking this on #691 and the DD support ticket we plan to create.