🌱 Add Singapore location and ap-southeast Network Zone

[apricote]

What this PR does / why we need it:

Add the new location sin (Singapore) and network zone ap-southeast to the CRDs + webhook validations.

The new location was released on 2024-08-06. Learn more at https://docs.hetzner.cloud/changelog#2024-08-06-new-location-singapore

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squash commits
• include documentation
• add unit tests

[apricote]

I dont really know why make verify fails (can reproduce locally). The output does not show any errors/warnings to me. I also did not add any links in this PR that could be broken.

https://github.com/syself/cluster-api-provider-hetzner/actions/runs/10262738794/job/28393040899?pr=1420#step:6:106

 make: *** [Makefile:747: lint-links] Error 2

[janiskemper]

@guettli this is another PR.. The linting fails for all right now. Thanks for the PR @apricote

[guettli]

I dont really know why make verify fails (can reproduce locally).

Don't worry about that. Our link-checker failed on http status 999 of LinkedIn. This will be fixed via #1421