docs: change the install section of all plugin readme.md to `This…

… plugin is included by default in Yarn4.`

packages/plugin-constraints/README.md

@@ -4,6 +4,4 @@ This plugin adds support for [constraints](https://yarnpkg.com/features/constrai
 
 ## Install
 
-```
-yarn plugin import constraints
-```
+This plugin is included by default in Yarn 4.

packages/plugin-exec/README.md

@@ -8,9 +8,7 @@ This plugin will add support to Yarn for the `exec:` protocol. This protocol is
 
 ## Install
 
-```
-yarn plugin import exec
-```
+This plugin is included by default in Yarn 4.
 
 ## Usage
 

packages/plugin-interactive-tools/README.md

@@ -4,9 +4,7 @@ This plugin adds support for various interactive commands.
 
 ## Install
 
-```
-yarn plugin import interactive-tools
-```
+This plugin is included by default in Yarn 4.
 
 ## Commands
 

packages/plugin-stage/README.md

@@ -4,6 +4,4 @@ This plugin adds support for the [`yarn stage`](https://yarnpkg.com/cli/stage) c
 
 ## Install
 
-```
-yarn plugin import stage
-```
+This plugin is included by default in Yarn 4.

packages/plugin-typescript/README.md

@@ -7,9 +7,7 @@
 
 ## Install
 
-```bash
-yarn plugin import typescript
-```
+This plugin is included by default in Yarn 4.
 
 ## Example
 

packages/plugin-version/README.md

@@ -4,6 +4,4 @@ This plugin adds support for the new [release workflow](https://yarnpkg.com/feat
 
 ## Install
 
-```
-yarn plugin import version
-```
+This plugin is included by default in Yarn 4.

packages/plugin-workspace-tools/README.md

@@ -4,9 +4,7 @@ This plugin adds support for various workspace-related commands.
 
 ## Install
 
-```
-yarn plugin import workspace-tools
-```
+This plugin is included by default in Yarn 4.
 
 ## Commands