update the Validators Admin-Guide (#2098)

### What
A community notice regarding the actions needed to prevent a failure-mode
with package upgrades.

### Why
We discovered a rare failure mode that occurs when stellar-core is upgraded
on top of a pending postgresql package update.

### Testing before merge
SDF SRE team  has tested and validated the procedure.

### Validation after merge
Website docs are updated

### Issue addressed by this PR
https://github.com/stellar/ops/issues/4337

Author: mwtzzz

docs/validators/admin-guide/maintenance.mdx

@@ -23,6 +23,24 @@ We recommend performing the following steps in order (repeat sequentially as nee
 4. When done, start your instance that should rejoin the network
 5. The instance will be completely caught up when it's both `Synced` and _there is no backlog in uploading history_.
 
+## Community Notice: Action Required Before Stellar Core Upgrade
+
+We have identified a specific scenario on Ubuntu where upgrading the Stellar Core package can inadvertently trigger an automatic, unplanned upgrade of PostgreSQL. This can occur if a newer version of PostgreSQL is available in your repositories. This is because the Stellar Core package lists it as a dependency, "apt" automatically installs the newer PostgreSQL package within the same transaction as the Stellar Core upgrade, which can lead to an unexpected database restart and can cause the Core process to crash.
+
+Action: To prevent service disruption and ensure data integrity, the node operators must manually apply PostgreSQL patch BEFORE executing the Stellar Core package upgrade.
+
+### Pre-Upgrade Steps
+
+Before running an apt command that includes the stellar-core package, follow these steps to manage the PostgreSQL upgrade safely:
+
+1. Gracefully shut down stellar-core: `sudo systemctl stop stellar-core`
+2. `apt list --upgradable | grep postgresql`
+3. `sudo apt-get install postgresql-16 postgresql-client-16 # or the specific version package`
+4. Restart stellar-core: `sudo systemctl start stellar-core`
+5. Wait for core to get in sync: `stellar-core-cmd info`
+
+Then, you may proceed to deploy the newer version of stellar-core package in your customary way (`apt`, config management tool, etc)
+
 ## Special Considerations During Quorum Set Updates
 
 When you join the ranks of node operators, it's also important to join the conversation. The best way to do that: follow the`#validators` channel on the [Stellar Developer Discord](https://discord.gg/stellardev). If you can't do that for some reason, sign up for the [Stellar Validators Google Group](https://groups.google.com/forum/#!forum/stellar-validators).