Changelog for 2.12.1 (#5220)

Downgrad min supported OSX version to 13 (#5219)

* Downgrade min supported OSX version to 13
* Version upgrade to v2.12.1

Downgraded min supported OSX version to 13

Changelog changes and some modification to the KIP-848 migration guide (

#5214)

* Changelog changes and some modification to the KIP-848 migration guide
* Add that KIP-848 is not enabled by default and other PR comments

[KIP-848] Added migration guide and removed preview warning (#5210)

[KIP-320] Validate assigned partitions before starting to consume fro…

…m them (#4931)

* Fetched committed offsets should be validated
before starting to consume from it.
Failing test and mock handler implementation
for returning the committed offset leader epoch
instead of current leader epoch.

* Validate the offsets before starting to fetch assigned partitions

* Add more test cases for partition assignment
offset validation

* Fix for test 0139 subtest `do_test_store_offset_without_leader_epoch` . When fetching an offset it returns the leader epoch used when committing, not the current
leader epoch.
Given the mock cluster fix the test needs to be changed.

* Fix test `0139` subtest `do_test_list_offsets_leader_change`:
use cloned partition list for listing offsets, to avoid the fake leader epoch is then used for validation when assigning.

Fix ListOffsets mock handler for logging the correct returned leader epoch.

* Changelog entry

* Reduce number of tests in quick mode

* Add a new fetch state when finishing validating and starting to seek after a truncation,
to avoid a second repeated validation and possibly duplicated messages.

* Increase single test timeout

* Fix to leave the group in `rd_kafka_cgrp_incr_unassign_done` if terminate was requested, as done in `rd_kafka_cgrp_unassign_done` and `rd_kafka_cgrp_consumer_incr_unassign_done`

* Mock cluster, set the group as empty when last member leaves
instead of triggering a rebalance

* Test 0139 with mock cluster marked as local.
Doesn't delete topic if tests are local only as it's
possible there's no cluster to connect to
and it speeds up completing the test

* Resume the partition before fetch start or before validation

Fix deadlock in `rd_kafka_reset_any_broker_down_reported` (#5156)

* Fix deadlock in `rd_kafka_reset_any_broker_down_reported` by releasing and reacquiring the broker lock, to respect lock ordering and avoid deadlocks
* Moved resetting the broker down reported field after setting the broker state for consistency with rest of the counters when unlocking

Fix deadlock in `rd_kafka_reset_any_broker_down_reported` (#5156)

* Fix deadlock in `rd_kafka_reset_any_broker_down_reported` by releasing and reacquiring the broker lock, to respect lock ordering and avoid deadlocks
* Moved resetting the broker down reported field after setting the broker state for consistency with rest of the counters when unlocking

librdkafka v2.11.1 (#5152)

Update C++ client error codes (#5134)