Downgrade to protobuf v25.5

It turns out `protobuf` v26.6 isn't compatibile with ScalaPB 0.9.8 used
by Scala 2.11:

```txt
Caused by: java.lang.UnsupportedOperationException:
  As of 2022/09/29 (release 21.7) makeExtensionsImmutable should not be
  called from protobuf gencode. If you are seeing this message, your
  gencode is vulnerable to a denial of service attack. You should
  regenerate your code using protobuf 25.6 or later. Use the latest
  version that meets your needs. However, if you understand the risks
  and wish to continue with vulnerable gencode, you can set the system
  property `-Dcom.google.protobuf.use_unsafe_pre22_gencode` on the
  command line.

See security vulnerability:
GHSA-h4h5-3hr4-j3g2
```

As mentioned in bazel-contrib#1710, we should consider dropping Scala 2.11 support at this
point, since there's no ScalaPB release for it that supports later versions of
`protobuf`. That, and we could remove some of the special case code added in
the following changes, amongst other 2.11 support details:

- bazel-contrib#1631
- bazel-contrib#1648
- bazel-contrib#1687
- bazel-contrib#1688

Author: mbland

scala/deps.bzl

@@ -43,9 +43,9 @@ def rules_scala_dependencies():
     maybe(
         http_archive,
         name = "com_google_protobuf",
-        sha256 = "ecfeb9f673e63321b4871c1cee6b5adeb0ef711d08a0c5bb1945271767df65bf",
-        strip_prefix = "protobuf-25.6",
-        url = "https://github.com/protocolbuffers/protobuf/archive/refs/tags/v25.6.tar.gz",
+        sha256 = "3cf7d5b17c4ff04fe9f038104e9d0cae6da09b8ce271c13e44f8ac69f51e4e0f",
+        strip_prefix = "protobuf-25.5",
+        url = "https://github.com/protocolbuffers/protobuf/archive/refs/tags/v25.5.tar.gz",
     )
 
     maybe(