k8s: fix MySQL-TLS schema references in deploy/job/cronjob

[TobiKr]

The database.tls input variable schema (k8s/variables.tf) declares six
attributes: enabled, config, server_name, ca_cert_key, cert_key, key_key.
However k8s/deploy.tf, k8s/job.tf, and k8s/cronjob.tf reference two
attributes that do not exist on that type:

• local.database.tls.cert_secret_key -> should be cert_key
• local.database.tls.secret_name -> not on tls; the MySQL secret
  is local.database.secret_name

These are inside dynamic blocks gated on local.database.tls.enabled,
but Terraform type-checks the expressions during planning regardless,
so any consumer setting database.tls.enabled = true (or even leaving
TLS off, since the type-check is static) hits:

Error: Unsupported attribute
This object does not have an attribute named "cert_secret_key".
Error: Unsupported attribute
This object does not have an attribute named "secret_name".

Align all three files with the variable schema, matching the pattern
already used for the password env var (which correctly reads
local.database.secret_name).

[BCTBB]

@TobiKr Thank you for your contribution! We'll schedule the review of your proposed changes for next sprint.