website: Drop support for GCC 5 and 6

GCC 5 and 6 were dropped in the following:
https://gem5-review.googlesource.com/c/public/gem5/+/47460
https://gem5-review.googlesource.com/c/public/gem5/+/47459

This page updates the website documentation.

Change-Id: Ib0f281b34069b05ee2b4e471955c896023a72f95
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/47461
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index d114967..a8ccfc8 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -37,7 +37,7 @@
 ## Dependencies
 
 * **git** : gem5 uses git for version control.
-* **gcc**: gcc is used to compiled gem5. **Version >=5 must be used**. We
+* **gcc**: gcc is used to compiled gem5. **Version >=7 must be used**. We
 support up to gcc Version 10.
 * **Clang**: Clang can also be used. At present, we support Clang 6 to
 Clang 9 (inclusive).
diff --git a/_pages/documentation/learning_gem5/part1/part1_1_building.md b/_pages/documentation/learning_gem5/part1/part1_1_building.md
index e300512..6cddce1 100644
--- a/_pages/documentation/learning_gem5/part1/part1_1_building.md
+++ b/_pages/documentation/learning_gem5/part1/part1_1_building.md
@@ -37,7 +37,7 @@
         sudo apt install git
         ```
 
-2.  gcc 5+
+2.  gcc 7+
     :   You may need to use environment variables to point to a
         non-default version of gcc.
 
@@ -47,7 +47,7 @@
         sudo apt install build-essential
         ```
 
-       **We support GCC Versions >=5, up to GCC 10**
+       **We support GCC Versions >=7, up to GCC 10**
 
 3.  [SCons 3.0+](http://www.scons.org/)
     :   gem5 uses SCons as its build environment. SCons is like make on