Closes gh-18715

Closes gh-18717

* pr/18705:
  Fix typos

Closes gh-18705

See gh-18705

* pr/18566:
  Rename `max-http-post-size` server property

Closes gh-18566

Rename `max-http-post-size` to `max-http-form-post-size` for Jetty and
Tomcat to make it clearer that they only apply to POSTed form content.

See gh-18566

* pr/18536:
  Polish "Upgrade to Jetty 9.4.21.v20190926"
  Upgrade to Jetty 9.4.21.v20190926

Closes gh-18536

See gh-18536

See gh-18536

Closes gh-18660

* pr/18604:
  Optimize debug level logs

Closes gh-18604

See gh-18604

* pr/18665:
  Polish

Closes gh-18665

See gh-18665

* gh-18648:
  Test the Gradle Plugin against Gradle 5.6.3

Closes gh-18648

See gh-18648

See gh-18473

Prior to this commit, all supported servers would share the same
configuration property `server.connection-timeout`. Unfortunately, in
many cases the behavior of this timeout changes depending on the server.
From actual connection setup timeout, to detecting and closing idle
connections, this property cannot be properly translated from one server
implementation to another.

This commit deprecates this configuration property and introduces server
specific properties:

* `server.jetty.connection-idle-timeout`
(Time that the connection can be idle before it is closed.)
* `server.netty.connection-timeout`
(Connection timeout of the Netty channel.)
* `server.tomcat.connection-timeout`
(Amount of time the connector will wait, after accepting a connection,
for the request URI line to be presented.)
* `server.undertow.no-request-timeout`
(Amount of time a connection can sit idle without processing a request,
before it is closed by the server.)

`server.connection-timeout` is now deprecated and will be removed in a
future release.

Fixes gh-18473

* pr/18612:
  Use LinkedHashSet for deterministic order in test assertion

Closes gh-18612

See gh-18612

Closes gh-18607

See gh-18229

* pr/18581:
  Polish "Add metadata entry for spring.main.register-shutdown-hook"
  Add metadata entry for spring.main.register-shutdown-hook

Closes gh-18581

See gh-18581

See gh-18581

* pr/18578:
  Polish "Clarify formatting recommendations for configuration property javadoc"
  Clarify formatting recommendations for configuration property javadoc

Closes gh-18578

See gh-18578

This commit adds a note to spring-boot-features.adoc to clarify that
using javadoc tags to format configuration property descriptions is not
supported.

See gh-18578

* pr/18580:
  Use LinkedHashSet for deterministic order in test assertions

Closes gh-18580

See gh-18580

* pr/18579:
  Rename tests to match Surefire expectations

Closes gh-18579

See gh-18579

* gh-18575:
  Make the logger static final

Closes gh-18575

See gh-18575

* pr/18546:
  Fix incorrect backslash escape in documentation sample

Closes gh-18546

See gh-18546

* gh-18539:
  Remove accidental usage of UnsupportedOptionException

Closes gh-18539

See gh-18539

Fixes gh-18410