Cleanup broken documentation links, update links which have changed, standardize link syntax

[jamesfredley]

This started by evaluating broken links in grails-doc, but expanded from there to correct the same URLs across the project. It updates grails.org URLs to grails.apache.org and updates all links, which can be changed, to https.

In some cases the best available URL was used to replace one that returned a 404. In the rare case where there was no URL available, like in the instance of a 15 year old blog post, the URL was removed.

It now represents around 1,000 fixed or updated URLs.

There are a few grails.util.* api links which still 404, due to #14978

Broken links were identified and verified as fixed using https://github.com/lycheeverse/lychee/releases/tag/lychee-v0.19.1.

grails-data and groovydocs were skipped and issues will be created to work on those broken URLs. #14990 & #14991

./gradlew grails-doc:build
cd \grails-doc\build\docs
lychee-x86_64-windows.exe . --format markdown --output results.md --exclude-path api --exclude-path grails-data --exclude-path grails-data/hibernate5/api --exclude ".*.css$" --exclude "^http://localhost" --exclude-path guide/pages

review results.md

Here is the summary from the latest run. https://dev.mysql.com/downloads/connector/j/ are false positives.

Summary

Status	Count
🔍 Total	97352
✅ Successful	96709
⏳ Timeouts	0
🔀 Redirected	0
👻 Excluded	637
❓ Unknown	0
🚫 Errors	6

## Errors per input

### Errors in .\guide\conf.html

* [ERROR] </grails-core/grails-doc/build/docs/api/grails/util/Environment.html> | Cannot find file
* [403] <https://dev.mysql.com/downloads/connector/j/> | Rejected status code (this depends on your "accept" configuration): Forbidden

### Errors in .\guide\plugins.html

* [ERROR] </grails-core/grails-doc/build/docs/api/grails/util/BuildScope.html> | Cannot find file

### Errors in .\guide\single.html

* [ERROR] </grails-core/grails-doc/build/docs/api/grails/util/BuildScope.html> | Cannot find file
* [ERROR] </grails-core/grails-doc/build/docs/api/grails/util/Environment.html> | Cannot find file
* [403] <https://dev.mysql.com/downloads/connector/j/> | Error (cached)

[jdaugherty]

Great Job!

My only concern is:

The license and notice files have to match exactly since it is spelled out that way in the licenses. We can change headers, etc, but I don't believe we can change the LICENSE and NOTICE files