zlib: bump minimum to 1.2.5.2 (was: 1.2.0.4)

1.2.5.2 was released on 2011-12-18. (vs. 1.2.0.4 on 2003-08-10)

It allows to:
- use `Z_BLOCK` unconditionally.
- use `inflateReset2()` to replace `inflateEnd()` + `inflateInit2()`
  and save a memory allocation.
- use `Z_CONST` and `z_const` (in a future commit).

Suggested-by: Dan Fandrich
Ref: https://github.com/curl/curl/pull/16142#discussion_r1985449743

Closes #16616

docs/INTERNALS.md

@@ -27,7 +27,7 @@ versions of libs and build tools.
  - OpenSSL      1.0.2a
  - LibreSSL     2.9.1
  - GnuTLS       3.1.10
- - zlib         1.2.0.4
+ - zlib         1.2.5.2
  - libssh2      1.2.8
  - c-ares       1.6.0
  - libssh       0.9.0