[16_0_X] Add 2026 PbPb eras

[stahlleiton]

PR description:

Backport of #49684

[cmsbuild]

A new Pull Request was created by @stahlleiton for CMSSW_16_0_X.

It involves the following packages:

• Configuration/Eras (operations)
• Configuration/PyReleaseValidation (pdmv)
• Configuration/StandardSequences (operations)

@AdrianoDee, @DickyChant, @antoniovagnerini, @cmsbuild, @davidlange6, @fabiocos, @ftenchini, @mandrenguyen, @miquork can you please review it and eventually sign? Thanks.
@GiacomoSguazzoni, @Martin-Grunewald, @VinInn, @VourMa, @dgulhan, @elusian, @fabiocos, @felicepantaleo, @makortel, @mmasciov, @mmusich, @mtosi, @rovere, @sameasy, @slomeo this is something you requested to watch as well.
@ftenchini, @mandrenguyen, @sextonkennedy you are the release manager for this.

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Pull request #49687 was updated. @AdrianoDee, @DickyChant, @Martin-Grunewald, @antoniovagnerini, @cmsbuild, @davidlange6, @fabiocos, @ftenchini, @mandrenguyen, @miquork, @mmusich can you please check and sign again.