[SYCL][Doc] Refresh info about releases (#20517)

Author: AlexeySachkov

README.md

@@ -53,15 +53,8 @@ See available Dockerfiles to create containers with pre-built/pre-installed DPC+
 
 #### Releases
 
-Daily builds of the sycl branch on Linux are available at
-[releases](https://github.com/intel/llvm/releases).
-A few times a year, we publish [Release Notes](./sycl/ReleaseNotes.md) to
-highlight all important changes made in the project: features implemented and
-issues addressed. The corresponding builds can be found using
-[search](https://github.com/intel/llvm/releases?q=oneAPI+DPC%2B%2B+Compiler&expanded=true)
-in daily releases. None of the branches in the project are stable or rigorously
-tested for production quality control, so the quality of these releases is
-expected to be similar to the daily releases.
+See [dedicated documentation](./sycl/doc/Releases.md) about various releases
+that we provide.
 
 #### Build from sources
 

sycl/doc/Releases.md

@@ -30,7 +30,10 @@ Release branches are prefixed with `sycl-rel-`
 ### _[Upcoming]_ `6.3.0` release
 
 This release will be made from the
-[`sycl-rel-6_3`](https://github.com/intel/llvm/tree/sycl-rel-6_3) branch.
+[`sycl-rel-6_3`](https://github.com/intel/llvm/tree/sycl-rel-6_3) branch and we
+have the following tags published:
+- [`v6.3.0-rc1`](https://github.com/intel/llvm/releases/tag/v6.3.0-rc1) -
+  release candidate 1 for 6.3.0 release
 
 ### **[Latest]** `6.2.0` release
 
@@ -41,14 +44,20 @@ we have the following tags published:
 - [`v6.2.0-rc1`](https://github.com/intel/llvm/releases/tag/v6.2.0-rc1) -
   release candidate 1 for 6.2.0 release
 
-### `6.1.0` release
+### [Legacy] releases
+
+These releases are _legacy_ meaning that we do not provide support for the
+corresponding release branches anymore, no bug fixes (functional or security)
+will be backported to them and no new tags will be made.
+
+#### `6.1.0` release
 
 This release was made from
 the [`sycl-rel-6_1_0`](https://github.com/intel/llvm/tree/sycl-rel-6_1_0)
 branch and we have the following tags published:
 - [`v6.1.0`](https://github.com/intel/llvm/releases/tag/v6.1.0)
 
-### `6.0.X` releases
+#### `6.0.X` releases
 
 These releases are made from
 the [`sycl-rel-6_0_0`](https://github.com/intel/llvm/tree/sycl-rel-6_0_0)