vtk: Update to 9.6.1

[Dave-Allured]

Description

• Update VTK 9.5.2 --> 9.6.1.
• Switch from Gitlab source, to Github mirror. Avoids Anubis blockers.
• Fix livecheck. Was previously broken by Anubis.

Type(s)

Tested on

CI only.

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?