docs/INSTALL.md: update min Go version to 1.18

In this commit, we update the min Go version to be version 1.18. This
breaks our prior rule of supporting the last two major versions, but
allows us to use the larger features in 1.18 sooner, such as the newly
released type parameters.

docs/INSTALL.md

@@ -93,23 +93,23 @@ following build dependencies are required:
 
 ### Installing Go
 
-`lnd` is written in Go, with a minimum version of 1.16. To install, run one of 
+`lnd` is written in Go, with a minimum version of 1.18. To install, run one of 
 the following commands for your OS:
 
 <details>
   <summary>Linux (x86-64)</summary>
   
   ```
-  wget https://dl.google.com/go/go1.17.1.linux-amd64.tar.gz
-  sha256sum go1.17.1.linux-amd64.tar.gz | awk -F " " '{ print $1 }'
+  wget https://dl.google.com/go/go1.18.linux-amd64.tar.gz
+  sha256sum go1.18.linux-amd64.tar.gz | awk -F " " '{ print $1 }'
   ```
 
   The final output of the command above should be
-  `dab7d9c34361dc21ec237d584590d72500652e7c909bf082758fb63064fca0ef`. If it
+  `e85278e98f57cdb150fe8409e6e5df5343ecb13cebf03a5d5ff12bd55a80264f`. If it
   isn't, then the target REPO HAS BEEN MODIFIED, and you shouldn't install
   this version of Go. If it matches, then proceed to install Go:
   ```
-  sudo tar -C /usr/local -xzf go1.17.1.linux-amd64.tar.gz
+  sudo tar -C /usr/local -xzf go1.18.linux-amd64.tar.gz
   export PATH=$PATH:/usr/local/go/bin
   ```
 </details>
@@ -118,12 +118,12 @@ the following commands for your OS:
   <summary>Linux (ARMv6)</summary>
   
   ```
-  wget https://dl.google.com/go/go1.17.1.linux-armv6l.tar.gz
-  sha256sum go1.17.1.linux-armv6l.tar.gz | awk -F " " '{ print $1 }'
+  wget https://dl.google.com/go/go1.18.linux-armv6l.tar.gz
+  sha256sum go1.18.linux-armv6l.tar.gz | awk -F " " '{ print $1 }'
   ```
 
   The final output of the command above should be
-  `ed3e4dbc9b80353f6482c441d65b51808290e94ff1d15d56da5f4a7be7353758`. If it
+  `a80fa43d1f4575fb030adbfbaa94acd860c6847820764eecb06c63b7c103612b`. If it
   isn't, then the target REPO HAS BEEN MODIFIED, and you shouldn't install
   this version of Go. If it matches, then proceed to install Go:
   ```