chore: Bump Buf ver installed from Makefile (#107)

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## This PR
- Bumps the version of the Buf tool installed by Makefile to latest

### Related Issues
- Current version of Buf (1.6.0) can't parse some of the template YAMLs
from this repo; e.g. the `plugin` entry is [not
available](https://github.com/bufbuild/buf/blob/25520a3fb15c8fab140e76e7238e6ace8dac13d9/private/buf/bufgen/bufgen.go#L270-L277)
under `plugins`, in version 1.6.0

### How to test
Running `make gen-java` works on my machine after the version bump

Signed-off-by: Fabrizio Demaria <fdema@spotify.com>

Makefile

@@ -1,4 +1,4 @@
-BUF_VERSION=v1.6.0
+BUF_VERSION=v1.26.1
 
 guard-%:
 	@ if [ "${${*}}" = "" ]; then \