[JENKINS-57528] - Load detached plugins which are implicit deps of other plugins at startup even w/o UC (#4000)

* Do not warn repeatedly of the same missing update site dependency.
This happens when locally testing split plugins (despite their presence in WEB-INF/detached-plugins/*.hpi).

(cherry picked from commit 918a0fd2211210a158279b2f19690c86801fe4a1)

* Detached plugins must be kept up to date with security advisories.

(cherry picked from commit e08d28a69897ff94811e02040255a9d266c44d0d)

* Load detached plugins which are implicit dependencies of other plugins at startup, even if the update center was not used.

(cherry picked from commit ad7a36001d30a6174b25e616fc3c215dd24919d1)

* Adjusted expected version after e08d28a69897ff94811e02040255a9d266c44d0d.

(cherry picked from commit 84f939cebd1a0d03633e7b7c01e71ac387fb6463)

* e08d28a69897ff94811e02040255a9d266c44d0d exposed tests which had been incorrectly storing plugins as *.hpi rather than *.jpi.

(cherry picked from commit a0abad7ec474b28d2d388f075bc54f961046b09a)

* Test failure after ad7a36001d30a6174b25e616fc3c215dd24919d1.

(cherry picked from commit 0084e8dbdddf93bb1353516c1fe20d807577631f)

* Another test failure after ad7a36001d30a6174b25e616fc3c215dd24919d1; fewer installDetachedPlugin calls are now required.

(cherry picked from commit b65bb54e9975a7c77103bf7cf4da82295498f308)

Signed-off-by: Nolivier lamy <olamy@apache.org>

[JENKINS-51577] - Windows Agent Installer: Enable TLS 1.2 on agent downloads when running with .NET 4.6 or above

Signed-off-by: Nolivier lamy <olamy@apache.org>

Signed-off-by: Nolivier lamy <olamy@apache.org>