Update wording & images around Etna upgrade (#1924)

* update wording

* update: replace subnet image with L1s

* fix: remove reference to previous subnet image

* fix: subnet to l1

* Update avalanche-l1s.mdx

Signed-off-by: Martin Eckardt <m.eckardt@outlook.com>

---------

Signed-off-by: Martin Eckardt <m.eckardt@outlook.com>
Co-authored-by: Andrea Vargas <43045203+Andyvargtz@users.noreply.github.com>
Co-authored-by: Martin Eckardt <m.eckardt@outlook.com>
Co-authored-by: Martin Eckardt <martin.eckardt@avalabs.org>

content/docs/avalanche-l1s/upgrade/customize-avalanche-l1.mdx

@@ -3,7 +3,7 @@ title: Customize an Avalanche L1
 description: Learn how to customize your EVM-powered Avalanche L1.
 ---
 
-All Avalanche L1s can be customized by utilizing [`Subnet Configs`](#avalanche-l1-configs).
+All Avalanche L1s can be customized by utilizing [`L1s Configs`](#avalanche-l1-configs).
 
 an Avalanche L1 can have one or more blockchains. For example, the Primary Network, which is an Avalanche L1, a special one nonetheless, has 3 blockchains. Each chain can be further customized using chain specific configuration file. See [here](/nodes/configure/configs-flags) for detailed explanation.
 

content/docs/protocol/avalanche-l1s.mdx

@@ -13,10 +13,8 @@ Avalanche's [Primary Network](/learn/primary-network) is a special Avalanche L1
 
 ![image](/images/subnet1.png)
 
-(Image adopted from [this article](https://www.coinbase.com/cloud/discover/dev-foundations/intro-to-avalanche-subnets))
-
 <Callout title="Note">
-Every validator in an Avalanche L1 **must** also validate the Primary Network.
+Every validator of an Avalanche L1 **must** sync the P-Chain of the Primary Network for interoperability.
 </Callout>
 
 Node operators that validate an Avalanche L1 with multiple chains do not need to run multiple machines for validation. For example, the Primary Network is an Avalanche L1 with three coexisting chains, all of which can be validated by a single node, or a single machine.