PHP 8 support

[jankonas]

Fixes #111

Checklist

• [x] Code follows coding conventions held in this repo
• [ ] ~Automated tests have been added~ (no code in src was changed)
• [x] Tests are passing
• [ ] ~Docs have been updated (if applicable)~ (support is stated as "PHP 7 and above", which covers PHP 8 too)
• [x] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

API reference can be generated by running vendor/bin/phpdoc

[codecov[bot]]

Codecov Report

Merging #112 (2f09861) into master (9fb7eb8) will decrease coverage by 0.00%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #112      +/-   ##
============================================
- Coverage     98.13%   98.12%   -0.01%     
- Complexity      195      200       +5     
============================================
  Files            22       23       +1     
  Lines           591      588       -3     
============================================
- Hits            580      577       -3     
  Misses           11       11              

Impacted Files	Coverage Δ
src/Kentico/Kontent/Delivery/DefaultMapper.php	98.43% <0.00%> (-0.03%)	:arrow_down:
src/Kentico/Kontent/Delivery/UrlBuilder.php	100.00% <0.00%> (ø)
...rc/Kentico/Kontent/Delivery/ContentTypeFactory.php	100.00% <0.00%> (ø)
...entico/Kontent/Delivery/Models/Taxonomies/Term.php	100.00% <0.00%> (ø)

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[Simply007]

Thank you for PR @jankonas,

I have briefly checked the changes and they look fine. I'll do a proper review next week and also try to validate the version release plan so that we don't need to release multiple major version close to each other.

[Simply007]

I have updated https://github.com/Kentico/kontent-delivery-sdk-php/wiki/Doing-PHP-stuff-on-Windows-with-VS-Code accordingly to PHP 8