Update repository image for gitea memcached to use bitnamilegacy/memcached

Update repository image for gitea memcached to use bitnamilegacy/memcached

Gitea helm chart relies on Bitnami images for memcached.

Beginning August 28th, 2025, Bitnami evolved its public catalog to offer
a curated set of hardened, security-focused images under the new Bitnami
Secure Images initiative. As part of this transition, starting August 28th,
all existing container images, including older or versioned tags have been
migrated from the public catalog (docker.io/bitnami) to the "Bitnami Legacy"
repository (docker.io/bitnamilegacy), where they will no longer receive updates.

Reference: https://github.com/bitnami/charts/tree/main/bitnami/memcached

Change-Id: I967f7d2daa88ccd091413fe9cfff25b715586b46
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>