Update README to link to v3 MTA Model documentation instead of v2?

[bmcdonn2]

The README links to the old v2 MTA spec which was last updated in 2017:

https://github.com/SAP/cloud-mta-build-tool/blob/62cd98bfa3ad21b79c86dad19c2676e6b9e3d590/README.md?plain=1#L20

Given that the mbt tool supports v3 wouldn't it make more sense to have that linked instead? The link to the newer model is here:

https://www.sap.com/documents/2021/09/66d96898-fa7d-0010-bca6-c68f7e60039b.html