Document upgrade procedure to Antrea v2

[antoninbas]

We are removing some Alpha APIs in Antrea v2. All changes are in line with our versioning policy: https://github.com/antrea-io/antrea/blob/main/docs/versioning.md

An example the v1alpha1/Tier CRD: #6162. One concern would be a user upgrading from an older Antrea version (e.g. 1.11) to Antrea v2.0. In this case, the v1beta1 version of API didn't exist in v1.11, so antctl upgrade api-storage cannot be used. My assumption would be that direct upgrade to v2.0 would not be possible in this case, and that users would need to upgrade to an intermediate version (1.13, 1.14, or 1.15) first. The fact that the 2 API resources are technically the "same" (conversion strategy is None) should be irrelevant here.

We should make sure that this is well-documented, and include a pointer to the relevant documentation in the Antrea v2.0 release notes.

See also #6162 (comment)

[antoninbas]

@luolanzone I should be able to submit a PR for this tomorrow. It should be a short document but I think it's important to have it for the v2 release.