Bump mxnet from 1.5.0 to 1.9.1 in /installation/Linux

Bumps [mxnet](https://github.com/apache/incubator-mxnet) from 1.5.0 to 1.9.1.
- [Release notes](https://github.com/apache/incubator-mxnet/releases)
- [Changelog](https://github.com/apache/mxnet/blob/master/NEWS.md)
- [Commits](apache/mxnet@1.5.0...1.9.1)

---
updated-dependencies:
- dependency-name: mxnet
  dependency-type: direct:production
...

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Author: dependabot[bot]

installation/Linux/requirements_cpu.txt

@@ -13,7 +13,7 @@ torch==1.2.0
 torchvision==0.4.0 
 tensorflow==1.12.0 
 keras==2.2.5 
-mxnet==1.5.0
+mxnet==1.9.1
 gluoncv==0.5
 tabulate
 netron

installation/Linux/requirements_tensorflow2_cpu.txt

@@ -13,7 +13,7 @@ torch==1.2.0
 torchvision==0.4.0 
 tensorflow==2.0.0 
 keras==2.3.1 
-mxnet==1.5.0
+mxnet==1.9.1
 gluoncv==0.5
 tabulate
 netron