[openmage/magento-lts] Fix for arbitrary command execution in custom layout update through blocks

Impact

Custom Layout enabled admin users to execute arbitrary commands via block methods.

References

• https://github.com/OpenMage/magento-lts/security/advisories/GHSA-c9q3-r4rv-mjm7
• https://github.com/OpenMage/magento-lts/releases/tag/v19.4.22
• https://github.com/OpenMage/magento-lts/releases/tag/v20.0.19
• https://nvd.nist.gov/vuln/detail/CVE-2021-39217
• https://github.com/OpenMage/magento-lts/commit/289bd4b4f53622138e3e5c2d2cef7502d780086f
• https://github.com/advisories/GHSA-c9q3-r4rv-mjm7