chore(lint): enable gosec (#7792)

Enable "gosec" linter.

Exclude:

- All G115 (integer overflow) findings, to be fixed separately.

Add targeted gosec annotations for:

- non-crypto math/rand usage
- md5 used only for file change detection
- G114 ("net/http serve with no timeout settings"), to be fixed
  separately.

Other findings fixed.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

.golangci.yml

@@ -11,6 +11,7 @@ linters:
     - copyloopvar
     - durationcheck
     - godoclint
+    - gosec
     - govet
     - ineffassign
     - intrange
@@ -39,7 +40,11 @@ linters:
       - path: _test\.go
         linters:
           - perfsprint
+          - gosec
   settings:
+    gosec:
+      excludes:
+        - G115
     govet:
       enable:
         - nilness