Use default keyword in Pages templates

What does this MR do and why?

Describe in detail what your merge request does and why.

From https://docs.gitlab.com/ee/ci/yaml/index.html#globally-defined-image-services-cache-before_script-after_script,

Defining image, services, cache, before_script, and after_script globally is deprecated. Support could be removed from a future release.

Use default instead.

ci: do not use deprecated globally-defined image, services, cache, before_script, and after_script

https://docs.gitlab.com/ee/ci/yaml/index.html#globally-defined-image-services-cache-before_script-after_script

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