docs: fix changelog headings (aws#1700)

v0.23.0 (aws#1665)

See CHANGELOG

v0.22.0 (aws#1518)

* v0.22.0

See CHANGELOG

chore: add changelog preamble (aws#1415)

Required for changelog-parser to be able to extract
the version descriptions during release to github.

chore: fix java app init template after bucket orphaning (aws#1352)

Since s3 buckets now have DeletionPolicy
of "Retain" by default, the init template's
expectation file must be updated.

Related aws#1273

v0.19.0 (aws#1281)

See CHANGELOG for details.

v0.18.1 (aws#1227)

See CHANGELOG

v0.18.0 (aws#1206)

See CHANGELOG

Also: fix toolkit diff integration test (output now contains a section header)

v0.17.0 (aws#1167)

See CHANGELOG

v0.16.0 (aws#1149)

See CHANGELOG