RUM-7106 refactor: Rename TTNS + ITNV to TNS + INV

[ncreated]

What and why?

📦 During documentation work, we decided that TNS (Time-to-Network-Settled) and INV (Interaction-to-Next-View) are better acronyms than TTNS and ITNV. Since these acronyms appeared in some public type names, this PR updates the naming accordingly

This change is not breaking because the updated APIs have not been released yet.

How?

A basic string replacement across the codebase.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes
• Add Objective-C interface for public APIs (see our guidelines [internal]) and run make api-surface)

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ncreated/RUM-7106/rename-ttns-and-itnv
Commit report: 7beaf27
Test service: dd-sdk-ios

✅ 0 Failed, 1082 Passed, 2665 Skipped, 1m 2.05s Total duration (1m 29.39s time saved)

[maxep]

Indeed better :)