Support for elasticsearch 8.15

[shemau]

Description

According to

ibmcloud cdb deployables              
Database Type:  elasticsearch
Version   Status   Preferred
8.15      stable   true
8.12      stable   false
8.10      stable   false
8.7       stable   false

8.15 is now available.

Reported via continuous test pipeline failure.

11619 showed

│ Error: Invalid value for variable
│ 
│   on [main.tf](http://main.tf/) line 21, in module "icd_elasticsearch":
│   21:   elasticsearch_version    = var.elasticsearch_version
│     ├────────────────
│     │ var.elasticsearch_version is "8.15"
│ 
│ Version must be 8.7, 8.10 or 8.12 (Enterprise or Platinum plan if 8.10 or
│ later).
│ 
│ This was checked by the validation rule at ../../variables.tf:20,3-13.
╵})

New or affected modules

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[ocofaigh]

There is already a PR open for this #318

[daniel-butler-irl]

Code updated and merged