fix: remove unsafe pkg usage from util.mempool

[na--]

What this PR does / why we need it:

Before this change, when I was running the tests with -race, the following error occurred:

fatal error: checkptr: unsafe.Slice result straddles multiple allocations

goroutine 60 gp=0xc0001ba8c0 m=5 mp=0xc000100008 [running]:
runtime.throw({0x14afd15?, 0xc0004e3d30?})
	runtime/panic.go:1067 +0x48 fp=0xc0004e3c18 sp=0xc0004e3be8 pc=0x4b0148
runtime.unsafeslicecheckptr(0x130a920, 0xc0002fa000, 0x40)
	runtime/unsafe.go:88 +0x65 fp=0xc0004e3c40 sp=0xc0004e3c18 pc=0x4a5b85
github.com/grafana/loki/v3/pkg/util/mempool.(*slab).get(0xc0002e5400, 0x10)
	github.com/grafana/loki/v3/pkg/util/mempool/pool.go:58 +0x25b fp=0xc0004e3d58 sp=0xc0004e3c40 pc=0x1299e7b
github.com/grafana/loki/v3/pkg/util/mempool.(*MemPool).Get(0xc0001a5f60, 0x10)
	github.com/grafana/loki/v3/pkg/util/mempool/pool.go:103 +0x146 fp=0xc0004e3e28 sp=0xc0004e3d58 pc=0x129a726
github.com/grafana/loki/v3/pkg/util/mempool.TestMemPool.func4(0xc00018d040)
	github.com/grafana/loki/v3/pkg/util/mempool/pool_test.go:63 +0x1f5 fp=0xc0004e3ee0 sp=0xc0004e3e28 pc=0x129b455
testing.tRunner(0xc00018d040, 0x14d9bd0)

And while the test can be fixed, I didn't see a reason to use unsafe in this package at all