Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Fix #74063: NumberFormatter fails after retrieval from session

While it would be desireable to actually support unserialization of
NumberFormatter instances, at least we should not allow

Fix #74063: NumberFormatter fails after retrieval from session

While it would be desireable to actually support unserialization of
NumberFormatter instances, at least we should not allow serialization
for now.

We also remove some doubtful tests, which have been added[1] claiming
that they would crash the intl extension, but apparently no fix has
been applied, and the test cases have not been marked as XFAIL.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=ed793b2a3f857fd49c0c1b036062140da5b3e674>

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