Add support for CSE 4.2.1

[adambarreiro]

This PR modifies the required elements to support CSE 4.2.1 in VCD:

• Adds a new example tfvars file for 4.2.1 with the new CAPVCD, CSI and CPI versions.
• Renames previous tfvars file as tfvars.example.4.2.0 for clarity.
• Modifies CAPVCD, CSI and CPI description in the documentation guide, also removes defaults values from example HCL to avoid mistakes.
• Adds a new section "Upgrade from CSE 4.2.0 to 4.2.1" with the new elements to add/update to the installation.
• Improves TestAccVcdCseKubernetesCluster destroy check phase, as ID is not always available.

Fixes:

• Creation of Kubernetes clusters failed when CSE Server configuration (VCDKEConfig) used certificates. A bump of the SDK fixes this issue (https://github.com/vmware/go-vcloud-director/pull/653)