build(deps): downgrade dependencies to 8.* versions for max compatibility

[Wwwsylvia]

What this PR does / why we need it

Dependencies in the 9.* series support .NET 8 and .NET 9, while 8.* supports .NET 6–8. As a library, sticking with 8.* ensures broader compatibility.

Which issue(s) this PR resolves / fixes

Please check the following list

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[Copilot]

Pull Request Overview

This PR downgrades package dependencies from the 9.* series to 8.* versions to improve compatibility. The goal is to support a broader range of .NET versions (.NET 6-8) rather than limiting support to .NET 8-9 only, which is appropriate for a library that aims for maximum compatibility.

Key changes:

• Downgraded Microsoft.Extensions.Caching.Abstractions from 9.0.9 to 8.0.0
• Downgraded Microsoft.Extensions.Caching.Memory from 9.0.9 to 8.0.1
• Downgraded System.Text.Json from 9.0.9 to 8.0.6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.84%. Comparing base (5cbb22c) to head (e41bf4b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #279   +/-   ##
=======================================
  Coverage   89.84%   89.84%           
=======================================
  Files          55       55           
  Lines        2392     2392           
  Branches      321      321           
=======================================
  Hits         2149     2149           
  Misses        161      161           
  Partials       82       82           

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[shizhMSFT]

LGTM