Update, remove unreachable update sites

During TXM Update command, if an updatesite is not reachable remove it and continue the Update command

Avoid this:

Looking for TXM updates...
Adding update site URL: https://gitlab.huma-num.fr/txm/txm-software/-/raw/master/dist//0.8.2/main/stable...
No stable level update available.
Stacktrace:
[1]  org.txm.rcp2.plugins.TXMUpdateHandler. addURL  TXMUpdateHandler.java, 495
[2] org.txm.rcp2.plugins.TXMUpdateHandler.addDefaultUpdateSites TXMUpdateHandler.java, 302
[3]  org.txm.rcp2.plugins.TXMUpdateHandler. execute  TXMUpdateHandler.java, 147
[4]                  org.txm.rcApplication. start  Application.java, 260
Error while adding artifact repo: https://gitlab.huma-num.fr/txm/txm-software/-/raw/master/dist//0.8.2/main/stable
Stacktrace:
[1]  org.txm.rcp2.plugins.TXMUpdateHandler. addURL  TXMUpdateHandler.java, 500
[2] org.txm.rcp2.plugins.TXMUpdateHandler.addDefaultUpdateSites TXMUpdateHandler.java, 302
[3]  org.txm.rcp2.plugins.TXMUpdateHandler. execute  TXMUpdateHandler.java, 147
[4]                  org.txm.rcApplication. start  Application.java, 260
org.eclipse.equinox.p2.core.ProvisionException: No repository found at https://gitlab.huma-num.fr/txm/txm-software/-/raw/master/dist//0.8.2/main/stable.
    at org.eclipse.equinox.internal.p2.repository.helpers.AbstractRepositoryManager.fail(AbstractRepositoryManager.java:398)
    at org.eclipse.equinox.internal.p2.repository.helpers.AbstractRepositoryManager.loadRepository(AbstractRepositoryManager.java:656)
    at org.eclipse.equinox.internal.p2.artifact.repository.ArtifactRepositoryManager.loadRepository(ArtifactRepositoryManager.java:119) 

(from redmine: issue id 3400, created on 2023/06/02 by Matthieu Decorde)