[IMP] SECURITY.md: deprecate version 15.0 support

closes odoo#192143

Signed-off-by: Florian Vranckx (flvr) <[email protected]>

Author: hamo-odoo

SECURITY.md

@@ -4,10 +4,10 @@
 
 | Version | Supported          |
 | ------- | ------------------ |
+| 18.0    | :white_check_mark: |
 | 17.0    | :white_check_mark: |
 | 16.0    | :white_check_mark: |
-| 15.0    | :white_check_mark: |
-| <=14.0  | :x:                |
+| <=15.0  | :x:                |
 
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