Remove links to old master branch (sigp#2190)

## Proposed Changes

In preparation for deleting the `master` branch, remove all links to it from the book/README.

.github/ISSUE_TEMPLATE.md

@@ -5,7 +5,7 @@ Please provide a brief description of the issue.
 ## Version
 
 Please provide your Lighthouse and Rust version. Are you building from
-`master`, which commit?
+`stable` or `unstable`, which commit?
 
 ## Present Behaviour
 

.github/workflows/test-suite.yml

@@ -3,7 +3,7 @@ name: test-suite
 on:
   push:
     branches:
-      - master
+      - stable
       - staging
       - trying
       - 'pr/*'

README.md

@@ -4,11 +4,11 @@ An open-source Ethereum 2.0 client, written in Rust and maintained by Sigma Prim
 
 [![Build Status]][Build Link] [![Book Status]][Book Link] [![Chat Badge]][Chat Link]
 
-[Build Status]: https://github.com/sigp/lighthouse/workflows/test-suite/badge.svg?branch=master
+[Build Status]: https://github.com/sigp/lighthouse/workflows/test-suite/badge.svg?branch=stable
 [Build Link]: https://github.com/sigp/lighthouse/actions
 [Chat Badge]: https://img.shields.io/badge/chat-discord-%237289da
 [Chat Link]: https://discord.gg/cyAszAh
-[Book Status]:https://img.shields.io/badge/user--docs-master-informational
+[Book Status]:https://img.shields.io/badge/user--docs-unstable-informational
 [Book Link]: https://lighthouse-book.sigmaprime.io
 [stable]: https://github.com/sigp/lighthouse/tree/stable
 [unstable]: https://github.com/sigp/lighthouse/tree/unstable

book/src/contributing.md

@@ -14,7 +14,7 @@ to work on.
 
 To start contributing,
 
-1. Read our [how to contribute](https://github.com/sigp/lighthouse/blob/master/CONTRIBUTING.md) document.
+1. Read our [how to contribute](https://github.com/sigp/lighthouse/blob/unstable/CONTRIBUTING.md) document.
 2. Setup a [development environment](./setup.md).
 3. Browse through the [open issues](https://github.com/sigp/lighthouse/issues)
    (tip: look for the [good first

book/src/intro.md

@@ -30,7 +30,7 @@ to understand how we develop and test Lighthouse.
 ## About this Book
 
 This book is open source, contribute at
-[github.com/sigp/lighthouse/book](https://github.com/sigp/lighthouse/tree/master/book).
+[github.com/sigp/lighthouse/book](https://github.com/sigp/lighthouse/tree/unstable/book).
 
-The Lighthouse CI/CD system maintains a hosted version of the `master` branch
+The Lighthouse CI/CD system maintains a hosted version of the `unstable` branch
 at [lighthouse-book.sigmaprime.io](http://lighthouse-book.sigmaprime.io).

book/src/local-testnets.md

@@ -4,5 +4,5 @@ During development and testing it can be useful to start a small, local
 testnet.
 
 The
-[scripts/local_testnet/](https://github.com/sigp/lighthouse/tree/master/scripts)
+[scripts/local_testnet/](https://github.com/sigp/lighthouse/tree/unstable/scripts)
 directory contains several scripts and a README that should make this process easy.

book/src/mainnet-validator.md

@@ -4,7 +4,7 @@
 [lh-book]: https://lighthouse-book.sigmaprime.io/
 [testnet-validator]: ./testnet-validator.md
 [advanced-datadir]: ./advanced-datadir.md
-[license]: https://github.com/sigp/lighthouse/blob/master/LICENSE
+[license]: https://github.com/sigp/lighthouse/blob/stable/LICENSE
 [slashing]: ./slashing-protection.md
 [discord]: https://discord.gg/cyAszAh
 

book/src/slashing-protection.md

@@ -177,4 +177,4 @@ out on Discord if you need help.
 The Lighthouse developers do not guarantee the perfect functioning of this software, or accept
 liability for any losses suffered. For more information see the [Lighthouse license][license].
 
-[license]: https://github.com/sigp/lighthouse/blob/master/LICENSE
+[license]: https://github.com/sigp/lighthouse/blob/stable/LICENSE