[8.x](backport #4373) Change "APM" to "Applications" when referencing the UI

[mergify]

Description

In 8.16, the Observability nav and Platform nav will both be updated to use "Applications" instead of "APM". Changes to the docs include:

• Instructions for navigating the UI: Update APM to Applications when instructing users to navigate the UI.
• APM UI → Applications UI: In Serverless docs, we use the term Applications UI instead of APM UI because the side navigation uses Applications. This PR aligns the stateful docs with the Serverless UI by changing references from APM UI to Applications UI.
• Images: I focused on screenshots that actually relate to APM/Applications. I don't think it's worth the effort to update screenshots that happen to show APM in the nav, but the point of the screenshot has nothing to do with APM. Let me know if you disagree!

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #4070

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4373 done by [Mergify](https://mergify.com).

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