Add Trino 421 release notes

docs/src/main/sphinx/release.md

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 ```{toctree}
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+release/release-421
 release/release-420
 release/release-419
 release/release-418

docs/src/main/sphinx/release/release-421.md

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+# Release 421 (6 Jul 2023)
+
+## General
+
+* Add support for check constraints in an `UPDATE` statement. ({issue}`17195`)
+* Improve performance for queries involving a `year` function within an `IN`
+  predicate. ({issue}`18092`)
+* Fix failure when cancelling a query with a window function. ({issue}`18061`)
+* Fix failure for queries involving the `concat_ws` function on arrays with more
+  than 254 values. ({issue}`17816`)
+* Fix query failure or incorrect results when coercing a
+  [structural data type](structural-data-types) that contains a timestamp. ({issue}`17900`)
+
+## JDBC driver
+
+* Add support for using an alternative hostname with the `hostnameInCertificate`
+  property when SSL verification is set to `FULL`. ({issue}`17939`)
+
+## Delta Lake connector
+
+* Add support for check constraints and column invariants in `UPDATE`
+  statements. ({issue}`17195`)
+* Add support for creating tables with the `column` mapping mode. ({issue}`12638`)
+* Add support for using the `OPTIMIZE` procedure on column mapping tables. ({issue}`17527`)
+* Add support for `DROP COLUMN`. ({issue}`15792`)
+
+## Google Sheets connector
+
+* Add support for {doc}`/sql/insert` statements. ({issue}`3866`)
+
+## Hive connector
+
+* Add Hive partition projection column properties to the output of
+  `SHOW CREATE TABLE`. ({issue}`18076`)
+* Fix incorrect query results when using S3 Select with `IS NULL` or
+  `IS NOT NULL` predicates. ({issue}`17563`)
+* Fix incorrect query results when using S3 Select and a table's `null_format`
+  field is set. ({issue}`17563`)
+
+## Iceberg connector
+
+* Add support for migrating a bucketed Hive table into a non-bucketed Iceberg
+  table. ({issue}`18103`)
+
+## Kafka connector
+
+* Add support for reading Protobuf messages containing the `Any` Protobuf type.
+  This is disabled by default and can be enabled by setting the 
+  `kafka.protobuf-any-support-enabled` configuration property to `true`. ({issue}`17394`)
+
+## MongoDB connector
+
+* Improve query performance on tables with `row` columns when only a subset of
+  fields is needed for the query. ({issue}`17710`)
+
+## Redshift connector
+
+* Add support for [table comments](/sql/comment). ({issue}`16900`)
+
+## SPI
+
+* Add the `BLOCK_AND_POSITION_NOT_NULL` argument convention. ({issue}`18035`)
+* Add the `BLOCK_BUILDER` return convention that writes function results
+  directly to a `BlockBuilder`. ({issue}`18094`)
+* Add the `READ_VALUE` operator that can read a value from any argument
+  convention to any return convention.  ({issue}`18094`)
+* Remove write methods from the BlockBuilder interface. ({issue}`17342`)
+* Change array, map, and row build to use a single `writeEntry`. ({issue}`17342`)