[3.x] TDB (iproto): review schema upgrade examples

[xuniq]

Related dev. issue(s): https://jira.vk.team/browse/TNTP-4863n https://jira.vk.team/browse/TNTP-4903 https://jira.vk.team/browse/TNTP-4902
Product: Tarantool DB
Since: 3.1
Root document: admin guide
SME: @ a1div0 @ Satbek

Details

MR to review:
https://gitlab.corp.mail.ru/tarantool/tdb/tarantooldb/-/merge_requests/173

New examples: migrations_schema_upgrade and tt_schema_upgrade

Follow-up MR: https://gitlab.corp.mail.ru/tarantool/tdb/tarantooldb/-/merge_requests/178