Update dependency phpstan/phpstan to v1.8.8

This MR contains the following updates:

Package	Type	Update	Change
phpstan/phpstan	require-dev	patch	1.8.7 -> 1.8.8

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Release Notes

phpstan/phpstan

v1.8.8

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Improvements 🔧

• Cover ExprPrinter by BC promise (#​1794), thanks @​staabm!
• Improve isset specification in falsy scope (#​1781), thanks @​herndlm!s
• Update phpstan/phpdoc-parser to 1.9.0 (https://github.com/phpstan/phpstan-src/commit/fd94186ee2a8065c94edb4603dd954584e9e8f45)

Bugfixes 🐛

• Do not drop original specified expression in TypeSpecifier (#​1793), #​8076, thanks @​herndlm!
• Preserve accessory types in MutatingScope::generalizeType (#​1732), #​8015, thanks @​herndlm!
• Specify types on get_parent_class() (#​1383), #​5552, thanks @​staabm!
• Do not load Composer\InstalledVersions to parse PHPStan version (https://github.com/phpstan/phpstan-src/commit/acbb55baeeb2f02b0ea2e61b4321e4f5dd5f65b2), #​8117

Function signature fixes 🤖

• Fix zadd argument order (#​1779), thanks @​araab!
• Fix zscan return types (#​1780), thanks @​araab!
• XMLWriter::startElementNS() $namespace arg is nullable (#​1772), thanks @​staabm!

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