Update logwatch ignore.conf to exclude Xapian messages about pending documents

[mmehnert]

Update logwatch ignore.conf to exclude Xapian messages about pending documents since it adds unnecessary noise to the logwatch summary emails.

• Improvement (non-breaking change that does improve existing functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary, I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md

[georglauterbach]

Just a small hint @mmehnert: when you write - [x], in GitHub markdown that becomes

• Check

which is quite a bit nicer than

• [ X ] Check

:)