Add security note to issue template

[jolheiser]

Added a note to the top of the issue template to send security concerns to the security email instead of opening a public issue.

I added it to its own comment block on the top so that it stands out more.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@062de8e). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #6281   +/-   ##
=========================================
  Coverage          ?   38.88%           
=========================================
  Files             ?      359           
  Lines             ?    50993           
  Branches          ?        0           
=========================================
  Hits              ?    19827           
  Misses            ?    28297           
  Partials          ?     2869

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