prepare 1.8.0 release

[pellared]

Changed

• profiles: drop gzip requirement. #661
• profiles: avoid optional keyword usage. #659
• profiles: make profile_id optional. #665
• profiles: use single Profile.sample_type and clarify use of timestamps. #649
• all: add notes about the attribute values restrictions. #683
  ⚠️ IMPORTANT: These restrictions can be dropped in a future minor release.
• profiles: clarify usage of the zero value as the first element of tables in ProfilesDictionary. #688, #698
• profiles: unsigned time_nanos and duration_nanos in Profile. #692
• profiles: improve attribute encoding in ProfilesDictionary. #672
• profiles: simplify profile stack trace representation. #708

Fixed

• examples: fix OTLP JSON Event example body. #666
• docs: minor specification fixes around UNAVAILABLE and RetryInfo. #669

Removed

• profiles: remove default_sample_type. #679
• profiles: remove has_* debug info fields, they are moving to attributes. #595
• profiles: remove Location.is_folded. #690

[Copilot]

Pull Request Overview

This PR prepares the 1.8.0 release by adding a new changelog entry with all the changes made since version 1.7.0.

• Updates CHANGELOG.md with version 1.8.0 release notes dated 2025-09-02
• Documents changes in three categories: Changed, Fixed, and Removed
• Includes comprehensive list of profile-related improvements and fixes with corresponding PR references

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[pellared]

@tigrannajaryan, ready for review.