PMM-14754 Probihibit removal of all pmm server nodes

[matejkubinec]

Ticket number: PMM-14754

Feature build: SUBMODULES-4477

Grafana PR: percona/grafana#916

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.96%. Comparing base (31318c7) to head (bb2cbe5).
⚠️ Report is 64 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5659      +/-   ##
==========================================
+ Coverage   43.59%   44.96%   +1.36%     
==========================================
  Files         415      218     -197     
  Lines       43134    27834   -15300     
==========================================
- Hits        18804    12515    -6289     
+ Misses      22454    13967    -8487     
+ Partials     1876     1352     -524     

Flag	Coverage Δ
admin	?
agent	?
managed	44.96% <100.00%> (+1.98%)	⬆️
vmproxy	?

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