[Docs] Incorporate the recent discussed changes to release process (#145285)

* Move up the date 2 weeks
* Make RC3 required
* Add more releases to the table to align with the release every two
weeks RFC.

This has been discussed in the following RFCs:

https://discourse.llvm.org/t/rfc-llvm-releases-every-two-weeks/84245

https://discourse.llvm.org/t/rfc-updating-and-aligning-the-llvm-release-process-before-llvm-21/86493

Author: tru

llvm/docs/HowToReleaseLLVM.rst

@@ -38,8 +38,8 @@ Releases should be tagged on Tuesdays.
 =============================== =========================
 Release                         Approx. Date
 =============================== =========================
-*release branch: even releases* *4th Tue in January*
-*release branch: odd releases*  *4th Tue in July*
+*release branch: even releases* *2nd Tue in January*
+*release branch: odd releases*  *2nd Tue in July*
 X.1.0-rc1                       3 days after branch.
 X.1.0-rc2                       2 weeks after branch.
 X.1.0-rc3                       4 weeks after branch
@@ -49,7 +49,11 @@ X.1.0-rc3                       4 weeks after branch
 **X.1.3**                       **12 weeks after branch**
 **X.1.4**                       **14 weeks after branch**
 **X.1.5**                       **16 weeks after branch**
-**X.1.6 (if necessary)**        **18 weeks after branch**
+**X.1.6**                       **18 weeks after branch**
+**X.1.7**                       **20 weeks after branch**
+**X.1.8**                       **22 weeks after branch**
+**X.1.9** (If necessary)        **24 weeks after branch**
+**Next release branches**       **~25 weeks after branch**
 =============================== =========================
 
 Release Process Summary
@@ -341,10 +345,10 @@ Below are the rules regarding patching the release branch:
    was created.  As with all phases, release managers and maintainers can reject
    patches that are deemed too invasive.
 
-#. *Before RC2* Patches should be limited to bug fixes or backend specific
+#. *Before RC2/RC3* Patches should be limited to bug fixes or backend specific
    improvements that are determined to be very safe.
 
-#. *Before RC3/Final Major Release* Patches should be limited to critical
+#. *Before Final Major Release* Patches should be limited to critical
    bugs or regressions.
 
 #. *Bug fix releases* Patches should be limited to bug fixes or very safe