Downgraded expected/common "BootstrapPackage not found" server error to a debug message.

[getvictor]

For #25265
Downgraded expected/common "BootstrapPackage not found" server error to a debug message. Occurs when UI/API checks if bootstrap package exists.

Checklist for submitter

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.85%. Comparing base (f7ac65d) to head (eafff54).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
ee/server/service/software_installers.go	0.00%	2 Missing ⚠️
server/service/apple_mdm.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25266   +/-   ##
=======================================
  Coverage   63.85%   63.85%           
=======================================
  Files        1616     1616           
  Lines      153829   153844   +15     
  Branches     3990     3990           
=======================================
+ Hits        98222    98237   +15     
- Misses      47793    47794    +1     
+ Partials     7814     7813    -1     

Flag	Coverage Δ
backend	64.72% <84.21%> (+<0.01%)	⬆️

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