Update serialization mechanism

[gmazzap]

Currently we have a rule that discourages the usage of magic __serialize and __unserialize while suggesting usage of the Serializable interface.

That was the "state of the art" up to PHP 7.3, and the rule even predates that.

With PHP 7.4 the Serializable interface is not recommended anymore (the RFC define it as "severely broken").

The new recommended method is to use __serialize and __unserialize methods.

Not being a major release PHP 7.4 did not deprecate Serializable.

PHP 8.1 triggers a deprecation warning if Serializable without also having __serialize and __unserialize.

And PHP 9 will probably remove the interface altogether.

Because v2 is PHP 7.4 we can safely discourage Serializable already and fully recommend __serialize and __unserialize.

This PR does that by:

• Edit the sniff that discouraged __serialize and __unserialize to actually suggest them where __sleep and __wakeup are used
• Add a sniff to discourage Serializable interface, suggesting __serialize and __unserialize

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes/features)
• [ ] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature.

What is the current behavior? (You can also link to an open issue here)

• __serialize and __unserialize are discouraged
• Serializable interface is suggested

What is the new behavior (if this is a feature change)?

• __serialize and __unserialize are suggested
• Serializable interface is discouraged

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

• New errors might be reported

Other information:

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[gmazzap]

We have to update documentation

Thank you @shvlv, done.