[3.x] refactor: Add initial backwards-compatible return types (#1763)

This is a first pass at adding backwards-compatible return types that we
can safely merge into the 3.x series, as we work towards #1744.

Return types are added to:

* public, protected and private methods in `final` classes
* private methods in `non-final` classes
* `final public` methods in `non-final` classes

The return types applied are based on:

* Rector's type coverage rules (up to level 21) - these are based on the
actual type returned where this can be conclusively determined from
static analysis.
* Manual modification to the phpdoc `@return` type of any interface the
method implements, if this was less specific than the type that Rector
detected. This is the type Rector would have used if the interface had a
strict return type, but we cannot add those until 4.x.

It also includes some improvements / corrections to invalid or 
old-style phpdoc `@return` tags for lists / arrays etc that phpstan
was previously ignoring but are now checked due to the presence
of a runtime return type.

Merge pull request #1775 from acoulton/next-3.x

docs: Add changelog for 3.28.0

Release 3.27.0

Add changelog for 3.26.0 release (#1683)

Add changelog for 3.25.0 release (#1670)

Add release notes for 3.24.1 (#1662)

Add release notes for 3.24.0 release (#1659)

Add changelog for 3.23.0 release (#1652)

chore: Add changelog for 3.16.1

Backports the compatibility fix for behat/gherkin for users who cannot
upgrade behat past 3.16.0.

Merge pull request #1634 from acoulton/next-3.x

Add changelog for 3.22.0 release