Path traversal

@backstage/backend-common is a common functionality library for backends for Backstage, an open platform for building developer portals. In @backstage/backend-common prior to versions 0.21.1, 0.20.2, and 0.19.10, paths checks with the resolveSafeChildPath utility were not exhaustive enough, leading to risk of path traversal vulnerabilities if symlinks can be injected by attackers. This issue is patched in @backstage/backend-common versions 0.21.1, 0.20.2, and 0.19.10.