Closes gh-20951

Closes gh-21068

Closes gh-20953

Fixes gh-21008
Co-authored-by: Andy Wilkinson <awilkinson@pivotal.io>

Closes gh-21170

As a follow-up for gh-21214, this commit renames the property to:

`"server.servlet.register-default-servlet=false"`

Closes gh-21214

This commit adds a new configuration property for configuring the
registration of the default Servlet in Servlet containers.

`"server.servlet.default-servlet.registered=false"`

The default of this property is still `true`, as it's been the case in
previous releases.

Closes gh-21214

* gh-21207:
  Polish "Stop limiting layer customization to external modules"
  Stop limiting layer customization to external modules

Closes gh-21207

See gh-21207

See gh-21207

* pr/21212:
  Fix some deprecations

Closes gh-21212

See gh-21212

Closes gh-20947

Closes gh-21255

Closes gh-21254

Closes gh-21253

Closes gh-21252

Closes gh-21251

Closes gh-21250

Closes gh-21249

Closes gh-21248

Closes gh-21247

Closes gh-21246

Closes gh-21245

Closes gh-21244

Closes gh-21243

Closes gh-21242

Closes gh-21241

Closes gh-21240

Closes gh-21239

Closes gh-21238

Closes gh-21237

Closes gh-21236

Closes gh-21235

Closes gh-21234

Closes gh-21233

Closes gh-21232

Closes gh-21231

Closes gh-21230

Closes gh-21229