maint: post-release administrivia

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* cfg.mk (old_NEWS_hash): Auto-update.

.prev-version

diff --git a/.prev-version b/.prev-version
index aaf18d2..897e56b 100644
--- a/.prev-version
+++ b/.prev-version
@@ -1 +1 @@
-3.7.5
+3.7.6

NEWS

diff --git a/NEWS b/NEWS
index 84d2723..ed264b2 100644
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,8 @@
 GNU Bison NEWS
 
+* Noteworthy changes in release ?.? (????-??-??) [?]
+
+
 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
 
 ** Bug fixes