[Synthetics] Allow user to rename private locations

[leweafan]

Describe the feature:

It'll improve user experience if private locations can be renamed.
Now private locations can not be renamed.

Describe a specific use case for the feature:

We created lot of synthetics tests and want to rename private locations to make it more obvious and user friendly.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[paulb-elastic]

Requires #156320