Bump ajv from 6.9.2 to 6.12.6 in /matrix-library

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.9.2 to 6.12.6. - [Release notes](https://github.com/ajv-validator/ajv/releases) - [Commits](ajv-validator/ajv@v6.9.2...v6.12.6) --- updated-dependencies: - dependency-name: ajv dependency-type: indirect ... Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]