message: Update security notice
  New information came to light after the original report, so this updates the
  notice to match the latest details.
diff: @@ -312,11 +312,12 @@ Changes in [3.15.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/
  ## Security notice
  -matrix-react-sdk 3.15.0 fixes a low severity issue (CVE-2021-21320) where the
  -user content sandbox can be abused to trick users into opening unexpected
  -documents. The content is opened with a `blob` origin that cannot access Matrix
  -user data, so messages and secrets are not at risk. Thanks to @keerok for
  -responsibly disclosing this via Matrix's Security Disclosure Policy.
  +matrix-react-sdk 3.15.0 fixes a moderate severity issue (CVE-2021-21320) where
  +the user content sandbox can be abused to trick users into opening unexpected
  +documents after several user interactions. The content can be opened with a
  +`blob` origin from the Matrix client, so it is possible for a malicious document
  +to access user messages and secrets. Thanks to @keerok for responsibly
  +disclosing this via Matrix's Security Disclosure Policy.
  ## All changes
  