[2021-12-06] drop support of php7.3

[jkowalleck]

on 2021-12-06 the support for php7.3 ended. see https://www.php.net/supported-versions.php

lets take this for a change to switch the min-version of this lib to 7.4 .
see https://www.php.net/manual/en/migration74.new-features.php

so to use the new language features, a breaking change will happen:

• drop php7.3 from CI
• drop php7.3 from composer manifest (php: ^7.4)
• migrate to php74 via php-cs-fixer
• make class properties type-save
• bump ergebnis/composer-normalize to a later version that does not need php7.3 - [ON HOLD] tools(deps-dev): update ergebnis/composer-normalize requirement from 2.18.0 to 2.22.0 in /tools/composer-normalize #46
• review the proposals below

---

PROPOSAL: remove getters and setters, if a property can be made type safe and converted to public.

PRO

• less methods to maintain

CON

• no setters = cannot use chained setters ala $instance->setA(1)->setB(2);

code changes

from

class C {
  /** @var bool */
  private $prop = true;

  /** @return $this */
  public function setProp(bool $prop): self
  {
     $this->prop = $prop;
     return $this;
  }
}

to

class C {
  public bool $prop = true;
}

---

[jkowalleck]

proposed change to remove getters/setters was dropped, for now.
removing setters wouod remove the feature to of chaining, which comes in handy in downstream implementations.
removing the getter might be appealing, but can be re-thought later.

see also: #128