Add input-log4j to skiplist

[karenzone]

Input-log4j was deprecated in 5.x. Recent restructuring in Beats documentation caused broken links in log4j. It doesn't make sense to release a new gem to a deprecated plugin. Instead, this PR adds input-log4j to the skiplist so that we won't pick up deprecations that I have to reverse each time we generate the plugin docs.

Related: #45 #50

Note to self @karenzone and reviewer(s): This updates one of three skiplists, and I hope it will solve the log4j regression problems. Other skiplists:

• Skiplist file - VPR: https://github.com/elastic/docs-tools/blob/master/versioned_plugins.rb
• Skiplist file - LSR: https://github.com/elastic/logstash/blob/master/rakelib/plugins-metadata.json

ToDo: Dig into other skiplist to see how they relate and if we can do some cleanup to avoid confusion. Tracked in issue #50.

[karenzone]

@yaauie, please take a look and comment and/or review

[jsvd]

LGTM