ZOOKEEPER-4191: Work around missing executable bits in source release tarball

symat noticed that the source tarball for 3.7.0rc0 is missing executable bits.

ztzg noticed that this can be worked around by reinstating the "old" version of the maven-assembly-plugin, which had been aligned in ZOOKEEPER-3833.

This patch implements the work around, and also applies cleanly on top of `branch-3.7.0` and `branch-3.7`.

Also discussed on dev:

  https://mail-archives.apache.org/mod_mbox/zookeeper-dev/202101.mbox/%3C875z3n9w75.fsf%40crosstwine.com%3E

Original report:

  https://mail-archives.apache.org/mod_mbox/zookeeper-dev/202101.mbox/%3cCAAMoRKLMf7tLosgqyiwYfFxXq-Zmiz=0oTGDijX5M=MHDF_JCgmail.gmail.com%3e

Author: Damien Diederen <dd@crosstwine.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>

Closes #1586 from ztzg/ZOOKEEPER-4191-missing-x-bits-source-tarball

pom.xml

@@ -690,6 +690,7 @@
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-assembly-plugin</artifactId>
+          <version>3.1.0</version>
         </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>