76834: sql: execute batch statements in an implicit transaction r=otan a=rafiss

fixes #44803
relates to #76490

Release justification: high value bug fix to existing functionality 

### *: prepare tests for batch stmt txn change

This commit will make the next one easier to review.

### sql: execute batch statements in an implicit transaction

Release note (bug fix): Previously statements that arrived in a batch
during the simple query protocol would all execute in their own implicit
transactions. Now, we match the Postgres wire protocol, so all these
statements share the same implicit transaction. If a BEGIN is included
in a statement batch, then the existing implicit transaction is
upgraded to an explicit transaction.

### sql: add session var for old implicit txn behavior

Release note (sql change): The enable_implicit_transaction_for_batch_statements
session variable was added. It defaults to true. When it is true,
multiple statements in a single query (a.k.a. a "batch statement") will
all be run in the same implicit transaction, which matches the Postgres
wire protocol. This setting is provided for users who want to preserve
the behavior of CockroachDB versions v21.2 and earlier.

77780: roachtest: update 22.1 version map to v21.2.7 r=celiala a=Azhng

Release note: None
Release justification: non-production code change.

77781: cloud: bump orchestrator to v21.2.7 r=celiala a=Azhng

Release note: None
Release justification: non-production code change.

77803: kvserverbase: move MaxCommandSizeDefault out of kvserver r=ajwerner a=ajwerner

```
$ bazel query "somepath(//pkg/sql, //pkg/kv/kvserver)"
INFO: Empty results
Loading: 0 packages loaded
```

Release justification: non-production code changes

Release note: None

Co-authored-by: Rafi Shamim <rafi@cockroachlabs.com>
Co-authored-by: Azhng <archer.xn@gmail.com>
Co-authored-by: Andrew Werner <awerner32@gmail.com>