See gh-34801

In commit 05ebca8, the `public` modifier was removed from the
SpringAtInjectTckTests class, which prevents it from being run as a
JUnit 3 test class.

To address that, this commit adds the missing `public` modifier as well
as a a code comment to help prevent this from happening again.

In addition, this commit updates spring-context.gradle to ensure that
the JUnit Vintage test engine is always applied. However, that Gradle
configuration is unfortunately ignored due to how our TestConventions
class has been implemented. Thus, that issue will have to be addressed
separately.

Closes gh-34800

(cherry picked from commit e384389)

Closes gh-34824

(cherry picked from commit 4172581)

(cherry picked from commit d0b186a)

As of gh-33806, the HttpComponents client request factory is forward
compatible with the 5.4+ versions of that library for configuring HTTP
request configuration.

This change would not tkae into account configuration set at the Spring
level because it would consider the default `RequestConfig` instance as
a custom one. This commit ensures that Spring sets its own configuration
if no custom configuration was set for all supported HttpComponents
generations.

Fixes gh-34854

… contract

Closes gh-34886

(cherry picked from commit 6a94444)

…ckstyle 10.23.1

Closes gh-34899

See: gh-34801

Closes gh-34801