Deprecate both PHP collectd monitors

[atoulme]

Description:
Deprecate collectd/opcache and collectd/php-fpm with a target removal of April 2026.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.23%. Comparing base (f6d932d) to head (3cd5d29).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...ignalfx-agent/pkg/monitors/collectd/monitorcore.go	0.00%	4 Missing ⚠️
...lfx-agent/pkg/monitors/collectd/opcache/opcache.go	0.00%	1 Missing ⚠️
...al/signalfx-agent/pkg/monitors/collectd/php/php.go	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (33.23%) is below the target coverage (35.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7140      +/-   ##
==========================================
+ Coverage   33.01%   33.23%   +0.21%     
==========================================
  Files         327      330       +3     
  Lines       23102    23325     +223     
==========================================
+ Hits         7628     7752     +124     
- Misses      14598    14680      +82     
- Partials      876      893      +17     

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