Update flex to v1.18.5 in composer.lock files for Symfony 4.0 and 5.0

[bwoebi]

Description

The newer flex version check against the existence of the manifest key instead of failing outright when it is missing.

This fixes our test_web_symfony runners.

Readiness checklist

• (only for Members) Changelog has been added to the release document.
• [ ] Tests added for this feature/bug.

Reviewer checklist

• Appropriate labels assigned.
• Milestone is set.
• Changelog has been added to the release document. For community contributors the reviewer is in charge of this task.

[labbati]

It is a mystery to me how this can fix the issue, since in theory we do composer update and not composer install (and we do not restore the vendor directory as far as I know). But if that work let's merge it.

[labbati]

What I just wrote is not true for symfony 4.0

dd-trace-php/Makefile

Lines 993 to 997 in e343bdb

	# We hit broken updates in this unmaintained version, so we committed a
	# working composer.lock and we composer install instead of composer update
	$(COMPOSER) --working-dir=tests/Frameworks/Symfony/Version_4_0 install
	php tests/Frameworks/Symfony/Version_4_0/bin/console cache:clear --no-warmup --env=prod
	$(call run_tests,tests/Integrations/Symfony/V4_0)

so go and ship it