Only include the security policy relevant to pg_tde

[jeltz]

SECURITY.md contained the policy for all our projects and not specifically pg_tde which is hardly useful information to the target audience.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.67%. Comparing base (523a300) to head (b24ac6d).
⚠️ Report is 2 commits behind head on main.

❌ Your project status has failed because the head coverage (59.67%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #502      +/-   ##
==========================================
- Coverage   59.69%   59.67%   -0.02%     
==========================================
  Files          67       67              
  Lines       10473    10473              
  Branches     1805     1805              
==========================================
- Hits         6252     6250       -2     
- Misses       3526     3528       +2     
  Partials      695      695              

Components	Coverage Δ
access	84.89% <ø> (ø)
catalog	87.93% <ø> (ø)
common	77.77% <ø> (ø)
encryption	71.56% <ø> (ø)
keyring	73.88% <ø> (ø)
src	94.20% <ø> (ø)
smgr	94.06% <ø> (ø)
transam	∅ <ø> (∅)

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