Move shipped proposals out of "future-releases". (#2983)

I also fixed any broken references to "future-releases" that I could
find within the language repo, including a few already-broken ones.

Author: munificent

accepted/2.12/abstract-external-fields/implementation-plan.md

@@ -5,7 +5,7 @@ Owner: [email protected] ([@eernstg](https://github.com/lrhn/) on GitHub)
 Relevant links:
 
 * [Tracking issue](https://github.com/dart-lang/sdk/issues/42560)
-* [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/future-releases/abstract-external-fields/feature-specification.md)
+* [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/2.12/abstract-external-fields/feature-specification.md)
 
 ## Phase 0 (Preliminary steps)
 

accepted/2.14/triple-shift-operator/implementation-plan.md

@@ -1,7 +1,7 @@
 # Implementation Plan for the `>>>` operator.
 
 Relevant documents:
-- [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/future-releases/triple-shift-operator/feature-specification.md)
+- [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/2.14/triple-shift-operator/feature-specification.md)
 ## Implementation and Release plan
 
 ### Phase 0 (Preliminaries)

accepted/future-releases/constructor-tearoffs/test-plan.md renamed to accepted/2.15/constructor-tearoffs/test-plan.md

@@ -2,7 +2,7 @@
 
 Relevant documents:
  - [Tracking issue](https://github.com/dart-lang/language/issues/113)
- - [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/future-releases/contravariant-superinterface-2018/feature-specification.md)
+ - [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/2.5/contravariant-superinterface-2018/feature-specification.md)
 
 
 ## Implementation and Release plan

accepted/future-releases/1847 - FinalizationRegistry/proposal.md renamed to accepted/2.17/1847-finalization-registry/proposal.md

@@ -57,7 +57,7 @@ It does so by:
 
 ### Package Default Language Version
 
-A language tool (like a compiler or analyzer) needs to derive this information for *all* packages available to the program it is processing. To this end, we record a language version for each available package in the new [`.dart_tool/package_config.json`](https://github.com/dart-lang/language/blob/master/accepted/future-releases/language-versioning/package-config-file-v2.md) file (which replaces `.packages` as the package configuration file), and this language version is used to define the *default* language version for all libraries in that package. Each individual library can override the default language version if necessary.
+A language tool (like a compiler or analyzer) needs to derive this information for *all* packages available to the program it is processing. To this end, we record a language version for each available package in the new [`.dart_tool/package_config.json`](https://github.com/dart-lang/language/blob/master/accepted/2.8/language-versioning/package-config-file-v2.md) file (which replaces `.packages` as the package configuration file), and this language version is used to define the *default* language version for all libraries in that package. Each individual library can override the default language version if necessary.
 
 The `.dart_tool/package_config.json` file is a JSON formatted text file which declares a number of named packages with a root and a package URI root directory for each. The SDK version for a specific package is added as an extra `languageVersion` property on the JSON object for the package.  The package configuration file is generated automatically by the `pub` tool as documented in the file’s specification.
 

accepted/future-releases/named-arguments-anywhere/feature-specification.md renamed to accepted/2.17/named-arguments-anywhere/feature-specification.md

@@ -5,7 +5,7 @@ Owner: [email protected] ([@lrhn](https://github.com/lrhn/) on GitHub)
 Relevant links:
 
 * [Tracking issue](https://github.com/dart-lang/language/issues/94)
-* [Proposal](https://github.com/dart-lang/language/blob/master/accepted/future-releases/language-versioning/feature-specification.md)
+* [Proposal](https://github.com/dart-lang/language/blob/master/accepted/2.8/language-versioning/feature-specification.md)
 
 ## Phase 0 (Prerequisite)