Upgrade the minimum required Go to 1.15

https://gitlab.com/gitlab-org/gitaly/-/merge_requests/3352

Changelog: fixed
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

doc/install/installation.md

@@ -242,7 +242,7 @@ sudo make install
 
 ## 3. Go
 
-In GitLab 8.0 and later, GitLab has several daemons written in Go. To install
+GitLab has several daemons written in Go. To install
 GitLab we need a Go compiler. The instructions below assume you use 64-bit
 Linux. You can find downloads for other platforms at the [Go download
 page](https://golang.org/dl).
@@ -251,11 +251,11 @@ page](https://golang.org/dl).
 # Remove former Go installation folder
 sudo rm -rf /usr/local/go
 
-curl --remote-name --progress-bar "https://dl.google.com/go/go1.13.5.linux-amd64.tar.gz"
-echo '512103d7ad296467814a6e3f635631bd35574cab3369a97a323c9a585ccaa569  go1.13.5.linux-amd64.tar.gz' | shasum -a256 -c - && \
-  sudo tar -C /usr/local -xzf go1.13.5.linux-amd64.tar.gz
+curl --remote-name --progress-bar "https://dl.google.com/go/go1.15.12.linux-amd64.tar.gz"
+echo 'bbdb935699e0b24d90e2451346da76121b2412d30930eabcd80907c230d098b7  go1.15.12.linux-amd64.tar.gz' | shasum -a256 -c - && \
+  sudo tar -C /usr/local -xzf go1.15.12.linux-amd64.tar.gz
 sudo ln -sf /usr/local/go/bin/{go,godoc,gofmt} /usr/local/bin/
-rm go1.13.5.linux-amd64.tar.gz
+rm go1.15.12.linux-amd64.tar.gz
 ```
 
 ## 4. Node