Rename variable to CI_SERVER_FQDN to match expected value

What does this MR do and why?

Following up on last week's !145950 (merged), I realized that this variable probably should be named CI_SERVER_FQDN to match all the similar variables. Because this variable's value is the server's FQDN, not the component's FQDN. It aligns with the other variables, for example:

Variable	Defined for	GitLab	Runner	Description
CI_COMPONENT_FQDN? 🤔	Pipeline	16.10	all	The fully qualified domain name (FQDN) of the instance. For example gitlab.example.com:8080.
CI_SERVER_FQDN 👍	Pipeline	16.10	all	The fully qualified domain name (FQDN) of the instance. For example gitlab.example.com:8080.
CI_SERVER_HOST	Pipeline	12.1	all	The host of the GitLab instance URL, without protocol or port. For example gitlab.example.com.
CI_SERVER_PORT	Pipeline	12.8	all	The port of the GitLab instance URL, without host or protocol. For example 8080.
CI_SERVER_PROTOCOL	Pipeline	12.8	all	The protocol of the GitLab instance URL, without host or port. For example https.
CI_SERVER_URL	Pipeline	12.7	all	The base URL of the GitLab instance, including protocol and port. For example https://gitlab.example.com:8080.
CI_SERVER_VERSION	Pipeline	all	all	The full version of the GitLab instance.

While it is being added to support the CI/CD components feature, CI_COMPONENT_FQDN does not accurately describe the content of the variable.

Related to https://gitlab.com/gitlab-org/technical-writing/-/issues/981

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