Bump to TF 2.5.0 (#1418)

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Author: yongtang

tensorflow_io/core/python/ops/version_ops.py

@@ -15,4 +15,4 @@
 """version_ops"""
 
 version = "0.18.0"
-require = "tensorflow>=2.5.0rc3,<2.6.0"
+require = "tensorflow>=2.5.0,<2.6.0"