gitattributes: improve GitHub and local git treatment of corpus and A…

…PI generated files. (envoyproxy#11742)

- Ignore v4alpha in GitHub diffs, since v4 is some ways away, reduce the amount of noise in reviews.
- Get rid of rules for .generated.pb_text, I don't think we have any of these today.
- Don't treat API generated files as binary locally, this causes pain when doing manual fixups on patches for CVEs, as they appear as binary diffs.

Signed-off-by: Harvey Tuch <htuch@google.com>

.gitattributes

@@ -1,10 +1,6 @@
 /docs/root/version_history/current.rst merge=union
-*.generated.pb_text linguist-generated=true
-*.generated.pb_text -diff -merge
+/api/envoy/**/v4alpha/* linguist-generated=true
 /generated_api_shadow/envoy/** linguist-generated=true
-/generated_api_shadow/envoy/** -diff -merge
 /generated_api_shadow/bazel/** linguist-generated=true
-/generated_api_shadow/bazel/** -diff -merge
 *.svg binary
 /test/**/*_corpus/* linguist-generated=true
-/test/**/*_corpus/* -diff -merge