Thread local storage and 2.5 support

[DNikolaevAtRocket]

Thread local storage is supported on v3.1+ nowadays.

The recent block allocator functionality uses anonymous mmap, and it's also supported by v3.1+ only.
Given that, the TLS emulation (zos-tls) can be safely removed unless somebody really needs DuckDB on v2.5 and is ready to do the mmap porting.

Are there any objections to the zos-tls removal from the upstream?

CC @v1gnesh, @IgorTodorovskiIBM

[v1gnesh]

🫡 go for it, Sir.
At least for DuckDB, my opinion is that whatever's required to keep this usable in zOS is more important than providing widest coverage.

[DNikolaevAtRocket]

Thanks! Will proceed with that.