2.28 -> 3.6 migration help

[mpg]

As 2.28 reaches end of life, we can expect more people migrating to 3.6 (the current LTS branch).

We have not fully addressed yet all know issues related to making most struct members private. We should:

• Encourage affected users to let us know so that we can properly support their use case in a future 3.6 release.
• Explain that MBEDTLS_PRIVATE is a work-around that is less dangerous in an LTS branch than in general (thanks to our ABI stability goals in LTS branches). A good place would be the existing 3.0 migration guide.
  This should be mentioned in the release announcement.

It would also be good to do a general quick pass over docs/3.0-migration-guide.md to check if anything needs updating.

[mpg]

It would also be good to do a general quick pass over docs/3.0-migration-guide.md to check if anything needs updating.

One thing that needs updating: the document is currently recommending mbedtls_ssl_conf_curves() which is deprecated in favour of mbedtls_ssl_conf_groups() since 3.1. Should probably mention conf_groups() only at this point (nobody should be using 3.0 as it's not maintained).