fix universal: fix random generator initialization in C++23

[emikg777]

utils::impl::RandomImpl initialized std::mt19937 via AsLvalue(impl::MakeSeedSeq()).

This fails to compile in C++23, because returning the rvalue-reference parameter from AsLvalue is affected by the C++23 implicit move rules from P2266R3. As a result, the returned expression may no longer be treated as a non-const lvalue reference, while the seed-sequence constructor of the generator expects a non-const lvalue seed sequence.

The error is (gcc 15.2.0):

error: cannot bind non-const lvalue reference of type ‘std::seed_seq&’ to an rvalue of type ‘std::seed_seq’

This change removes the AsLvalue helper and initializes the generator from a local std::seed_seq object instead. The local object stays alive for the duration of the generator construction, and no reference to a temporary is returned.

Testing:

• Built a userver-based project with CMAKE_CXX_STANDARD=23.
• Built and ran userver-universal-unittest.
• Full userver-universal-unittest run: 1671/1672 tests passed. The only failure was LogFilepath.UserverCroppedCorrectly, which appears to be path-layout-sensitive when userver is built as a CPM dependency under _deps/userver-src.