[linux installer script] Deprecate Fluentd support

[crobert-1]

Description:

Add notes that the functionality has been deprecated in help output as well as log a message when the CLI option is used.

Testing:

$ ./install.sh --help
...
Fluentd [DEPRECATED]:
...
$ ./install.sh --with-fluentd
[WARNING] Fluentd support has been deprecated and will be removed in a future release. Please use native OTel receivers instead (e.g. the filelog receiver).
Please enter your Splunk access token:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.75%. Comparing base (87bee7a) to head (2dcfef3).
Report is 1 commits behind head on main.

❌ Your project check has failed because the head coverage (34.75%) is below the target coverage (40.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (87bee7a) and HEAD (2dcfef3). Click for more details.

HEAD has 32 uploads less than BASE

Flag	BASE (87bee7a)	HEAD (2dcfef3)
	35	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6264       +/-   ##
===========================================
- Coverage   44.91%   34.75%   -10.16%     
===========================================
  Files         390      345       -45     
  Lines       27006    25122     -1884     
===========================================
- Hits        12129     8732     -3397     
- Misses      14009    15801     +1792     
+ Partials      868      589      -279     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[crobert-1]

lychee is failing on links unrelated to this PR, we should fix separately.