Update zendframework/zend-diactoros requirement from ^2.0 to ^2.1

[dependabot-preview]

Updates the requirements on zendframework/zend-diactoros to permit the latest version.

Release notes

Sourced from zendframework/zend-diactoros's releases.

zend-diactoros 2.1.3

Added

• Nothing.

Changed

• Nothing.

Deprecated

• Nothing.

Removed

• Nothing.

Fixed

• #363 modifies detection of HTTPS schemas via the $_SERVER['HTTPS'] value
  such that an empty HTTPS-key will result in a scheme of http and not
  https.

Changelog

Sourced from zendframework/zend-diactoros's changelog.

2.1.3 - 2019-07-10

Added

• Nothing.

Changed

• Nothing.

Deprecated

• Nothing.

Removed

• Nothing.

Fixed

• #363 modifies detection of HTTPS schemas via the $_SERVER['HTTPS'] value
  such that an empty HTTPS-key will result in a scheme of http and not
  https.

2.1.2 - 2019-04-29

Added

• Nothing.

Changed

• Nothing.

Deprecated

• Nothing.

Removed

• Nothing.

Fixed

• #355 adds phpdbg to the list of accepted non-SAPI enviornments for purposes
  of calling UploadedFile::moveTo().

2.1.1 - 2019-01-05

Added

... (truncated)

Commits

• 2797237 release: 2.1.3 readiness
• 1a3db8a Fix more CS issues
• 8d5aca5 Apply changes to comply with CodeReview
• 46dbee3 Add test to verify Scheme retrieval is correct
• 0c4e083 Modify detection of HTTPS
• 9a4b4a3 Bumped to next dev version (2.1.3)
• 37bf68b docs: opt-in to new version switcher
• 3fc4522 Adds CHANGELOG entry for #355
• 7fa95ce PHPDBG is also a valid non-sapi environment
• d966a8a Bumps changelog version to 2.1.2
• Additional commits viewable in compare view

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