os-bind: Deleting an entry doesn't increase the serial

[koelle25]

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Describe the bug
When removing a DNS entry from a zone in the BIND plugin, the zone serial doesn't change.

When used in combination with zone transfers, the secondary server(s) therefore get(s) out of sync and the deleted entry still resolves on those.

Interestingly, the BIND of the primary nameserver (the OPNsense instance) does NOT resolve the entry anymore, although the serial didn't change.

To Reproduce
Steps to reproduce the behavior:

1. Check zone prior to doing any changes:

   $ dig +short SOA example.com @ns1.example.com
   ns1.example.com. admin.example.com. 2507041206
   
   $ dig +short SOA example.com @ns2.example.com
   ns1.example.com. admin.example.com. 2507041206
   

2. Go to Services > Bind > Configuration > Primary Zones
3. Create an entry (e.g. test A 0.0.0.0) and save
4. Check zone:

   $ dig +short SOA example.com @ns1.example.com
   ns1.example.com. admin.example.com. 2507041209
   
   $ nslookup test.example.com ns1.example.com
   Name:   test.example.com
   Address: 0.0.0.0
   
   $ dig +short SOA example.com @ns2.example.com
   ns1.example.com. admin.example.com. 2507041209
   
   $ nslookup test.example.com ns2.example.com
   Name:   test.example.com
   Address: 0.0.0.0
   

5. Delete the test entry and save
6. Check zone again:

   $ dig +short SOA example.com @ns1.example.com
   ns1.example.com. admin.example.com. 2507041209
   
   $ nslookup test.example.com ns1.example.com
   ** server can't find test.idial.fh-dortmund.de: NXDOMAIN
   
   $ dig +short SOA example.com @ns2.example.com
   ns1.example.com. admin.example.com. 2507041209
   
   $ nslookup test.example.com ns2.example.com
   Name:   test.example.com
   Address: 0.0.0.0
   

   See that the serial hasn't changed and the results are different for primary/secondary nameserver.

Expected behavior
Any edit to a zone should change its serial, including deleting entries.

Screenshots
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Relevant log files
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Additional context
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Environment
OPNsense 25.1.10 (amd64) with os-bind 1.33_1 (containing bind920 version 9.20.10).

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